WO1988002226A1

WO1988002226A1 - Adhesive fingernail cover

Info

Publication number: WO1988002226A1
Application number: PCT/US1987/002519
Authority: WO
Inventors: Edward B. Richman; Philip E. Miner; William A. Herring
Original assignee: Avery International Corporation
Priority date: 1986-10-02
Filing date: 1987-10-02
Publication date: 1988-04-07

Abstract

A kit (10) of preformed fingernail covers (11) with a unique selection of sizes and duplications that affords an excellent fit for a large majority of women users, while limiting the total number of pieces required to reduce cost, simplify selection, and minimize waste. The individual covers (11) on their opposite ends each provide two different sizes. To assist the fitting process, the covers are displayed on a flexible, transparent card (12) in an anatomical manner.

Description

ADHESIVE FINGERNAIL COVER

BACKGROUND OF THE INVENTION

This invention relates to cosmetic products, and more particularly to improvements in decorative fingernail covering.

Prior Art

U.S. Patents 2,633,139 to Pettey and 2,746,460 to Jellinek, for example, disclose preformed nail covers marketed on a display card. This type of cover offers convenience to the user over common nail polish products. The preformed covers avoid painstaking brush application of polish, solvent odors, risk of spilling or other accidental contact of surfaces with polish and the relatively long time required for drying liquid polish to a hard finish.

A basic problem associated with preformed covers is the difficulty of fitting the nails of all women users with a single "universal" package of nails or a very few set of packages, e.g., petite and regular. The problem is not simply solved to satisfaction by providing a relatively large number of covers in each package because such a plethora of pieces can confuse the user or manicurist, lead to the perception of waste of unused pieces by the consumer, and increase manufacturing costs. SUMMARY OF THE INVENTION

The invention provides a package kit of preformed fingernail covers which, by a unique distribution of width sizes, offers a near-perfect fit for a large majority of women, in the order of nine out of ten, while limiting the total number of pieces allocated to a package.

The invention is based on discoveries made in connection with random sampling of the actual sizes of women's nails. Surprisingly, while no strong correlation appears between the size of a woman in terms of height, weight, etc. and her fingernails, the available data suggests definite practical upper and lower limits to a woman's fingernail size. In accordance with the invention, the range between these upper and lower limits can be divided into a limited number of sizes preferably differing successively by a uniform dimensional increment. Still further, study has revealed that a surprising number of women, in excess of 40%, have four or more nails which are perfectly fit by a single one of the possible alloted sizes. In accordance with the invention, the sizes of nail cover units included in a single package kit are duplicated in a unique manner to satisfactorily fit a desired high percentage of women users. In general, the intermediate sizes have approximately twice the duplication rate as that of the smaller and larger sizes. In the preferred embodiment, the number of sizes is chosen to be 10 and the dimensional increment between sizes is 1/32 inch. Where the sizes are duplicated in accordance with the disclosed unique selection, the resultant kit can be expected- to provide a good fit for 92% of women users matching the fingernail characteristics of the subjects sampled in the aforementioned study.

A nail cover unit of ideal width size for a particular nail when centered can at most reveal only 1/64 inch of the nail at each of its sides, yielding what can be considered a perfect fit. A nail exposure of 1/64 inch is essentially imperceptible and it is difficult for one of. ordinary dexterity to manually position a cover with greater positional accuracy. Nail covers which are one unit or 1/32 inch smaller than the ideal nail cover can at most reveal only 1/32 inch at each side of the cover, and even this exposure does not appear to be objectionable to the user. The use of a nail cover of a size one increment less than the ideal is referred to as "down-sizing."

Preferably each nail cover piece comprises two units, one at each of its ends and each of different size. This doublet construction reduces the total number of pieces required in a single package kit. Reduction of the number of pieces lowers the potential for confusion of the person choosing sizes of covers, reduces cost, and avoids waste perceived by the consumer. The disclosed size selection makes it possible to closely fit a great majority of women with no more than 20 pieces, and preferably with a set of 16 pieces, so that not more than one-half, and preferably considerably less than one-half, of the pieces will be unused after fitting both hands. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a kit of preformed nail covers packaged on a display card in accordance with the invention;

FIG. 2 is a plan view of the card of FIG. 1 illustrating a preferred selection and presentation of sizes;

FIG. 3 is an enlarged fragmentary view of a typical nail cover unit illustrating details of its geometry;

FIG. 4 illustrates the geometry of 10 sizes of units in a manner similar to FIG. 3;

FIG. 5 is a diagram or histogram illustrating frequency vs. nail width typical of a random group of adult women; and

FIG. 6 is a table of nail cover size duplication for the preferred selection displayed on the card of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, FIG. 1 illustrates a kit 10 of fingernail covers 11 on a display card 12. Ideally, the covers 11 are formed by die-cutting them from a web. The covers 11 are releasably retained on the card by a pressure-sensitive adhesive 13. When a cover 11 is stripped from a. card 12, the adhesive 13 remains with the cover and is used to adhere the cover to a fingernail. The covers 11 are made of acrylate, urethane, vinyl, or other suitable plastic film material which is flexible and somewhat stretchable. The pressure-sensitive adhesive 13 is acrylate or other conventional material used in adhesive tapes or bandages. The nail cover pieces 11 are arranged on the card 12 in a regular pattern of even number, with lateral and longitudinal spacing 16, 17, with reference to the lengthwise direction of the card 12. This arrangement allows the covers 11 to be economically die-cut from a web and stripped of a matrix at the spaces 16, 17 at high line speeds. Normally, the covers or pieces 11 are opaque and have a decorative color. Graphics, multiple colors, metallic glitter, and other special effects may be incorporated in the cover pieces 11. The card 12 is preferably a semirigid but flexible, transparent plastic film of .002-inch polyester such as that marketed under the trademark Mylar, for example. The illustrated card 12 is rectangular and of sufficient size to provide a margin 18 completely around the array of cover pieces 11 of a width of the same order of magnitude as the spaces 16, 17. FIG. 2 illustrates the preferred covers to an exacting relative scale. FIG. 5 is a diagram or histogram derived from actual measurements of the fingernails, grouping both right and left hands, of a large random sampling of adult women's hands taken in different geographic areas in the United States. For a statistically valid, large number of women, FIG. 5 indicates the frequency or number of nails of a given width size as a function of absolute width plotted on the abscissa. Examination of FIG. 5 reveals that practical limits exist for the small and large width sizes of women's nails. For example, these practical limits, from FIG. 5, can be considered as 20/64 inch at the minor width size and 38/64 inch at the major width size. Still further, from FIG. 5 a span of 38/64 minus 20/64 or 18/64 exists within the practical limits between small size nails and large size nails. From this comes the realization that the span of size differences can be bridged by a relatively small number of successive sizes and such successive sizes need only change in dimension a relatively small increment.

It can also be discovered from FIG. 5 that there are not great differences between the size of the center three fingers, i.e., the index, middle, and ring fingers, between women. Further, study of the data used to produce FIG. 5 reveals that when the fingers of both hands are considered, most women individually have at least three central fingers of essentially the same size, and over forty percent individually have at least four central fingers of the same size.

Based on the foregoing observations, it has been discovered that a good, or even perfect, fit can be offered to a large majority of women in a kit of a limited number of fingernail cover pieces with an appropriate series of sizes and proper duplication of specific sizes.

The table of FIG. 6 illustrates a series of sizes and the number of duplications of such sizes in a preferred selection offered on the card 12. Size 1 is taken as 20/64 and successive sizes are increased by a dimensional increment of 1/32 inch up to size 10, which is 38/64. A significant relation, set out in the table of FIG. 6, is that the intermediate sizes are duplicated in the preferred selection to a substantially greater degree than are the small and large sizes. Specifically, in the preferred selection, the average number of duplications for the intermediate sizes is 4.4 per size, while the average number of duplications for the small and for the large sizes is 2. Significantly, a kit containing the duplication of sizes set out in the table of FIG. 6 can be expected to provide a perfect or good fit for 92% of a group of women having the characteristics of those in the random study depicted in FIG. 5. A perfect fit exists where the kit provides sizes of nail covers that fit all of the nails of a woman with no more than 1/64 inch exposure on each side of the cover (e.g., a total of 1/32 inch difference between the nail cover and actual nail). A good fit is one in which, out of a total of 10 nails, generally only one or two need be fitted with a nail cover that is one size smaller than the optimum. Surveys have shown that this downsize option is not objectionable to most women when only one or a few nails are involved. In fact, the acceptability of the downsize option is the reason for the skew in the table of FIG. 6, where the intermediate sizes are indicated to include size 2 and exclude size 7. The nail covers 11 are preferably double-ended so that at each end there is a unit 21 contributing to the selection of usable sizes . The covers 11 are sufficiently long, e.g., in the order of 1-1/4 inch from end to end, to cover a typical long fingernail but too short to cover two such fingernails. The two end units 21 of each cover 11 are of different sizes with respect to each other. This dual size of a cover 11 provides greater utilization than would be available where each end of a cover had the same size, since the latter situation would only offer one size choice. The limited number of covers 11 provided by the present invention lessens potential confusion to the user by reducing the number of available selections, reduces the material cost and limits potential waste of unused covers.

FIG. 3 shows the geometrical development of a profile of a size 1 nail cover unit 21a. The shape of the unit 21a is established by two lines 22 diverging at an angle of 10 degrees, 5 degrees on each side of a central longitudinal axis 23. The root width, identified in the table of FIG. 6, is represented by a line 24 transverse to the longitudinal axis 23, where its ends fit on the diverging lines 22. The end of the cover, corresponding to the base of the fingernail, has a convex, curvilinear edge 26 described by portions of ellipsis. A first ellipse portion 27 is taken in a 40-degree plane and has a major axis of 5/8 inch. The arcuate ends of the first ellipse 27 are faired or rounded into the diverging lines 22 by short portions 28 of a second ellipse in a 40-degree plane with a major axis of 9/16 inch. The divergent lines 22 can be approximated by slightly convex lines. As illustrated in FIG. 2, the size 1 unit 21a is paired with a size 2 unit 21b. The divergent lines 22a of the size 1 unit 21a intercept analogous lines 22b of the size 2 unit 21b. The points of interception of these lines 22a, b are determined by setting an apex 29a, b of the respective ellipsoid edges 26a, b at the spacing given above at 1-1/4 inch.

FIG. 4 demonstrates the preferred geometrical development of sizes 1 through 10, which are generally typified by that explained above. It will be understood that the measure of each size unit is taken at the root or base line 24 substantially where the convex edge 26 merges with the diverging side edges 22. It should also be understood that the actual measurement of the fingernails for the data of the histogram of FIG. 5 was taken in the curved plane of the measured Tingernails. As seen in FIG. 4, the ellipsoid angle of the edges 26 is increased in the larger sizes, i.e., 7 through 9, as compared to the smaller sizes 1 through 6, so that these edges are flatter. Size 10 is also flatter at the base edge 26 being described by the short axis of a 30-degree ellipse. This increasing flatness in the larger sizes provides a better fit with female human fingernails. Preferably, each of the end units 21 is paired with an end unit of a size that differs by one size increment. The covers 11 are arranged on the card 12 in a manner analogous to the natural distribution of sizes on a person's hands, with the larger sizes, corresponding to the thumbs, at the center of the distribution, the intermediate sizes, corresponding to the central fingers, outward of the larger sizes, and the smaller sizes, corresponding to the pinkies or small fingers, outwardmost. The transparency and flexibility of the card 12 and spacing between covers 11 on the card facilitate selection of the proper size by allowing the card to be wrapped over a fingernail so that a prospective cover is directly over the fingernail and a fit can be closely judged. When a unit 21 has been selected as a matching fit, the associated fall cover 11 is peeled from the card 12 and the selected unit is applied to the fingernail. The unit or end opposite the selected unit is trimmed off to conform to the profile of the underlying fingernail.

While the disclosure sets forth an exemplary selection of nail cover sizes measured in English units, it will be understood that other dimensional units such as those of the metric system can be used. Some variation in the number of sizes available as well as the total number of pieces or units is also possible. In general, with 10 or about 10 sizes of widths, at least 14 double-ended pieces are necessary in a kit to provide a good fit for approximately two-thirds of an adult female population, this size majority being requisite from a marketing standpoint. A kit of more than 19 or 20 double-ended pieces does not afford a significant improvement in fit, and is undesirable from a marketing standpoint because the consumer is faced with the prospect of discarding more than half of the pieces.

It should be evident that this disclosure is by way of example and that various changes in addition to those discussed may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

FOR THEPURPOSES OF INFORMATION ONLY

CodesusedtoidentifyStatespartytothePCTonthefrontpagesofpamphletspublishinginternationalapplications under the PCT.

AT Austria FR France ML Mali

AU Australia GA Gabon MR Mauritania

BB Barbados GB United Kingdom MW Malawi

BE Belgium HU Hungary NL Netherlands

BG Bulgaria IT Italy NO Norway

BJ Benin JP Japan RO Romania

BR Brazil KP Democratic People's Republic SD Sudan

CF Central African Republic ofKorea SE Sweden

CG Congo KR Republic ofKorea SN Senegal

CH Switzerland LI Liechtenstein SU Soviet Union

CM Cameroon LK Sri Lanka TD Chad

DE Germany, Federal Republic of LU Luxembourg TG Togo

DK Denmark MC Monaco US United States ofAmerica

FI Finland MG Madagascar

ADHESIVE FINGERNAIL COVER

BACKGROUND OF THE INVENTION

Prior Art

The invention is based on discoveries made in connection with random sampling of the actual sizes of women's nails. Surprisingly, while no strong correlation appears between the size of a woman in terms of height, weight, etc. and her fingernails, the available data suggests definite practical upper and lower limits to a woman's fingernail size. In accordance with the invention, the range between these upper and lower limits can be divided into a limited number of sizes preferably differing successively by a uniform dimensional increment.

Still further, study has revealed that a surprising number of women, in excess of 40%, have four or more nails which are perfectly fit by a single one of the possible alloted sizes. In accordance with the invention, the sizes of nail cover units included In a single package kit are duplicated in a unique manner to satisfactorily fit a desired high percentage of women users. In general , the intermediate sizes have approximately twice the duplication rate as that of the smaller and larger sizes. In the preferred embodiment, the number of sizes is chosen to be 10 and the dimensional increment between sizes is 1/32 inch. Where the sizes are duplicated in accordance with the disclosed unique selection, the resultant kit can be expected to provide a good fit for 92% of women users matching the fingernail characteristics of the subjects sampled in the aforementioned study.

A nail cover unit of ideal width size for a particular nail when centered can at most reveal only 1/64 inch of the nail at each of its sides, yielding what can be considered a perfect fit. A nail exposure of 1/64 inch is essentially imperceptible and it is difficult for one of ordinary dexterity to manually position a cover with greater positional accuracy. Nail covers which are one unit or 1/32 inch smaller than the ideal nail cover can at most reveal only 1/32 inch at each side of the cover, and even this exposure does not appear to be objectionable to the user. The use of a nail cover of a size one increment less than the ideal is referred to as "down-sizing."

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, FIG. 1 illustrates a kit 10 of fingernail covers 11 on a display card 12. Ideally, the covers 11 are formed by die-cutting them from a web. The covers 11 are releasably retained on the card by a pressure-sensitive adhesive 13. When a cover 11 is stripped from a card 12, the adhesive 13 remains with the cover and is used to adhere the cover to a fingernail. The covers 11 are made of acrylate, urethane, vinyl, or other suitable plastic film material which is flexible and somewhat stretchable. The pressure-sensitive adhesive 13 is acrylate or other conventional material used in adhesive tapes or bandages. The nail cover pieces 11 are arranged on the card 12 in a regular pattern of even number, with lateral and longitudinal spacing 16, 17, with reference to the lengthwise direction of the card 12. This arrangement allows the covers 11 to be economically die-cut from a web and stripped of a matrix at the spaces 16, 17 at high line speeds. Normally, the covers or pieces 11 are opaque and have a decorative color. Graphics, multiple colors, metallic glitter, and other special effects may be incorporated in the cover pieces 11. The card 12 is preferably a semirigid but flexible, transparent plastic film of .002-inch polyester such as that marketed under the trademark Mylar, for example. The illustrated card 12 is rectangular and of sufficient size to provide a margin 18 completely around the array of cover pieces 11 of a width of the same order of magnitude as the spaces 16, 17. FIG. 2 illustrates the preferred covers to an exacting relative scale. FIG. 5 is a diagram or histogram derived from actual measurements of the fingernails, grouping both right and left hands, of a large random sampling of adult women's hands taken in different geographic areas in the United States. For a statistically valid, large number of women, FIG. 5 indicates the frequency or number of nails of a given width size as a function of absolute width plotted on the abscissa. Examination of FIG. 5 reveals that practical limits exist for the small and large width sizes of women's nails. For example, these practical limits, from FIG. 5, can be considered as 20/64 inch at the minor width size and 38/64 inch at the major width size. Still further, from FIG. 5 a span of 38/64 minus 20/64 or 18/64 exists within the practical limits between small size nails and large size nails. From this comes the realization that the span of size differences can be bridged by a relatively small number of successive sizes and such successive sizes need only change in dimension a relatively small increment.

The table of FIG. 6 illustrates a series of sizes and the number of duplications of such sizes in a preferred selection offered on the card 12. Size 1 is taken as 20/64 and successive sizes are increased by a dimensional increment of 1/32 inch up to size 10, which is 38/64. A significant relation, set out in the table of FIG. 6, is that the intermediate sizes are duplicated in the preferred selection to a substantially greater degree than are the small and large sizes. Specifically, in the preferred selection, the average number of duplications for the intermediate sizes is 4.4 per size, while the average number of duplications for the small and for the large sizes is 2. Significantly, a kit containing the duplication of sizes set out in the table of FIG. 6 can be expected to provide a perfect or good fit for 92% of a group of women having the characteristics of those in the random study depicted in FIG. 5. A perfect fit exists where the kit provides sizes of nail covers that fit all of the nails of a woman with no more than 1/64 inch exposure on each side of the cover (e.g., a total of 1/32 inch difference between the nail cover and actual nail). A good fit is one in which, out of a total of 10 nails, generally only one or two need be fitted with a nail cover that is one size smaller than the optimum. Surveys have shown that this downsize option is not objectionable to most women when only one or a few nails are involved. In fact, the acceptability of the downsize option is the reason for the skew in the table of FIG. 6, where the intermediate sizes are indicated to include size 2 and exclude size 7. The nail covers 11 are preferably double-ended so that at each end there is a unit 21 contributing to the selection of usable sizes. The covers 11 are sufficiently long, e.g., in the order of 1-1/4 inch from end to end, to cover a typical long fingernail but too short to cover two such fingernails . The two end units 21 of each cover 11 are of different sizes with respect to each other. This dual size of a cover 11 provides greater utilization than would be available where each end of a cover had the same size, since the latter situation would only offer one size choice. The limited number of covers 11 provided by the present invention lessens potential confusion to the user by reducing the number of available selections, reduces the material cost and limits potential waste of unused covers.

FIG. 3 shows the geometrical development of a profile of a size 1 nail cover unit 21a. The shape of the unit 21a is established by two lines 22 diverging at an angle of 10 degrees, 5 degrees on each side of a central longitudinal axis 23. The root width, identified in the table of FIG. 6, is represented by a line 24 transverse to the longitudinal axis 23, where its ends fit on the diverging lines 22. The end of the cover, corresponding to the base of the fingernail, has a convex, curvilinear edge 26 described by portions of ellipsis. A first ellipse portion 27 is taken in a 40-degree plane and has a major axis of 5/8 inch. The arcuate ends of the f irst ellipse 27 are faired or rounded into the diverging lines 22 by short portions 28 of a second ellipse in a 40-degree plane with a major axis of 9/16 inch. The divergent lines 22 can be approximated by slightly convex lines. As illustrated in FIG. 2, the size 1 unit 21a is paired with a size 2 unit 21b. The divergent lines 22a of the size 1 unit 21a intercept analogous lines 22b of the size 2 unit 21b. The points of interception of these lines 22a, b are determined by setting an apex 29a, b of the respective ellipsoid edges 26a, b at the spacing given above at 1-1/4 inch.

FIG. 4 demonstrates the preferred geometrical development of sizes 1 through 10, which are generally typified by that explained above. It will be understood that the measure of each size unit is taken at the root or base line 24 substantially where the convex edge 26 merges with the diverging side edges 22. It should also be understood that the actual measurement of the fingernails for the data of the histogram of FIG. 5 was taken in the curved plane of the measured fingernails. As seen in FIG. 4, the ellipsoid angle of the edges 26 is increased in the larger sizes, i.e., 7 through 9, as compared to the smaller sizes 1 through 6, so that these edges are flatter. Size 10 is also flatter at the base edge 26 being described by the short axis of a 30-degree ellipse. This increasing flatness in the larger sizes provides a better fit with female human fingernails. Preferably, each of the end units 21 is paired with an end unit of a size that differs by one size increment. The covers 11 are arranged on the card 12 in a manner analogous to the natural distribution of sizes on a person's hands, with the larger sizes, corresponding to the thumbs, at the center of the distribution, the intermediate sizes, corresponding to the central fingers, outward of the larger sizes, and the smaller sizes, corresponding to the pinkies or small fingers, outwardmost. The transparency and flexibility of the card 12 and spacing between covers 11 on the card facilitate selection of the proper size by allowing the card to be wrapped over a fingernail so that a prospective cover is directly over the fingernail and a fit can be closely judged. When a unit 21 has been selected as a matching fit, the associated full cover 11 is peeled from the card 12 and the selected unit is applied to the fingernail. The unit or end opposite the selected unit is trimmed off to conform to the profile of the underlying fingernail.

While the disclosure sets forth an exemplary selection of nail cover sizes measured in English units, it will be understood that other dimensional units such as those of the metric system can be used. Some variation in the number of sizes available as well as the total number of pieces or units is also possible. In general, with 10 or about 10 sizes of widths, at least 14 double-ended pieces are necessary in a kit to provide a good, fit for approximately two-thirds of an adult female population, this size majority being requisite from a marketing standpoint. A kit of more than 19 or 20 double-ended pieces does not afford a significant improvement in fit, and is undesirable from a marketing standpoint because the consumer is faced with the prospect of discarding more than half of the pieces.

Claims

WHAT IS CLAIMED IS:

1. A kit of fingernail covers comprising a selection of preformed units, the units being provided in a multiplicity of different width sizes ranging progressively through small, intermediate, and large classes, the number of duplicate units for each of the sizes in the immediate class averaging generally in the order of twice the number of duplicate units for each of the sizes in the small and large classes.

2. A kit as set forth in claim 1, wherein the number of intermediate sizes is generally equal to the combined total of the number of small and large sizes.

3. A kit as set forth in claim 2, wherein the sizes differ progressively from one to the next by a fixed increment.

4. A kit as set forth in claim 3, wherein the fixed increment is in the order of 1/32 inch.

5. A kit as set forth in claim 4, wherein the number of sizes is 10.

6. A kit as set forth in claim 5, wherein size 1 is generally equal to 20/64 inch and size 10 is generally equal to 38/64 inch.

7. A kit as set forth in claim 1, wherein said nail covers have opposite ends each providing a potentially usable unit.

8. A kit as set forth in claim 2, wherein more sizes in the large class are provided than in the small class.

9. A kit as set forth in claim 8, wherein the small class has one size, the intermediate class has five sizes, and the large class has four sizes.

10. A kit as set forth in claim 1, wherein said covers are releasably adhesively attached to a display card.

11. A kit as set forth in claim 10, wherein the display card is transparent.

12. A kit as set forth in claim 10, wherein the large size covers are positioned in a central portion of the card, the intermediate size covers are distributed on both sides and adjacent said large size covers, and the small size covers are distributed on both sides of intermediate size covers remote from the large size covers.

13. A kit as set forth in claim 10, wherein the covers are provided in an even number of pieces.

14. A kit as set forth in claim 13, wherein each cover piece comprises a pair of units each of different size.

15. A kit as set forth in claim 14, wherein the difference in sizes of paired units is generally one integer.

16. A kit as set forth in claim 1, wherein between 14 and 20 cover pieces are provided, each cover piece providing two units of different size.

17. A fingernail decoration product comprising an assorted array of between 14 and 20 flat self-adhesive flexible stretchable fingernail covering pieces releasably supported on a flat card, at least one end of each piece being bilaterally symmetrical with respect to its own longitudinal axis of symmetry and having at least one pair of opposed straight sides tapering toward each other to a root width as they approach said end of the piece, said opposed straight sides being joined at said end of the pieces by a convex curvilinear bilaterally symmetrical end edge, each of said opposed straight sides being at an angle of generally about 10 degrees with the longitudinal axis of symmetry of the piece, said root widths for the pieces in the array each being selected from one of ten sizes varying at regular intervals from 10/32 inch ± 1/64 inch to 19/32 inch ± 1/64 inch.