A linear polyamide is made by heating an aliphatic diamine with arylene dipropionic acids, or their derivatives, having the structural formula R1-(CH2)2-R-(CH2)2-R1, where R is an arylene radical, and R1 is COOH, CONH2, COCl or CN. The oxa- and thiaderivatives of the radical R may be used. Heating, preferably of an initially-formed salt, is continued in an inert atmosphere until the product has an intrinsic viscosity of 0.2 to 1.5, the temperature being increased periodically to keep the mass liquid. Filaments may be formed as by extrusion of the melted polyamide into a dry atmosphere or a liquid bath, and cold-drawn to between 300 and 600 per cent. The permanent set, or residual elongation at the breaking point, of these filaments is lower than in other types of polyamide, being 8.2 and 8.6 per cent in two examples. They are thus useful for fabrication with rubber as in the making of tyres, where it is desirable to have fibres with a residual elongation of less than 10 per cent. The filaments may be spun and woven into fabrics or used in the manufacture of brushes and the like or thin sheets may be extruded or solution-cast for use as wrapping films. Examples of acids are p-phenylene-, and 1,2-naphthalene-dipropionic acid and R in the above formula may be o, m and p-phenylene; the diphenylenes; the naphthylenes; the toluylenes; and their alkyl-substituted derivatives and the corresponding anthracene; phenanthrene; diphenyl and other aromatic hydrocarbon radicals. Acids derived from oxa- and thia-hydrocarbons such as diphenyl ether; diphenyl sulphide; alkoxybenzenes; and mixed aryl alkyl sulphides may be used, as may derivatives of the acids such as amides, acid anhydrides, acid chlorides, esters and nitriles. Examples of diamines are hexamethylene and decamethylene diamine; and compounds containing oxygen and sulphur such as triglycol diamine and di-(3-aminopropyl)-sulphide and their homologues. Arylene dipropionic acids may be prepared by the malonic ester synthesis from compounds of structure CH3-R-CH3 by halogenation to (BrCH2)2R, followed by condensation with ethyl malonate to give a condensed ester [(C2H5OOC)2CH-CH2]2R, which is hydrolysed to the corresponding acid, the latter being then pyrolized to split out CO2 and form the arylene dipropionic acid. They may also be prepared by Claisen condensations from halogen compounds of the type Br2-CHR-CH-Br2 which are hydrolysed to the dialdehyde OHC-R-CHO which is then condensed with ethyl acetate to give (C2H5OOC-CH=CH)2R, the latter being then hydrogenated to saturate the double bond, and hydrolysed. In specific examples polyamides from the salt of decamethylene diamine, or of hexamethylene diamine, and p-phenylene dipropionic acid, are compared in properties with polyamides from the same amines and p-phenylene diacetic acid. The Specification as open to inspection under Sect. 91 comprises also the use of acids of structure R1-(CH2)x-R-(CH2)x-R1 where x is a whole number greater than 1 and preferably from 2 to 6. This subject-matter does not appear in the Specification as accepted.ALSO:In making a linear polyamide (see Group IV (a)), an aliphatic diamine is heated with arylene dipropionic acids, or their derivatives, having the structural formula <FORM:0627733/IV (b)/1> where R is an arylene radical and R1 is COOH, CONH2, COCl or CN. The oxa- and thia-derivatives of the radical R may be used. Examples of acids are p-phenylene- and 1.2-naphthalene-dipropionic acid and R in the above formula may be o, m and p-phenylene; the diphenylenes; the naphthylenes; the toluylenes; and their alkyl-substituted derivatives and the corresponding anthracene; phenanthrene; diphenyl and other aromatic hydrocarbons radicals. Acids derived from oxa- and thia-hydrocarbons such as diphenyl ether; diphenyl sulphide; alkoxybenzenes; and mixed aryl alkyl sulphides may be used, as may derivatives of the acids such as amides, acid anhydrides, acid chlorides, esters and nitriles. Arylene dipropionic acids may be prepared by the malonic ester synthesis from compounds of structure CH3-R-CH3 by halogenation to (Br CH2)2 R followed by condensation with ethyl malonate to give a condensed ester [(C2H5OOC)2CH-CH2]2R which is hydrolysed to the corresponding acid, the latter being then pyrolized to split out CO2 and form the arylene dipropionic acid. They may also be prepared by Claisen condensations from halogen compounds of the type Br2-CH-R-CH-Br2 which are hydrolysed to the dialdehyde OHC-R-CHO which is then condensed with ethyl acetate to give (C2H5OOC-CH=CH)2 R the latter being then hydrogenated to saturate the double bond, and hydrolysed. In specific examples polyamides from the salt of decamethylene diamine, or of hexamethylene diamine, and p-phenylene dipropionic acid, are compared in properties with polyamides from the same amines and p-phenylene diacetic acid. The Specification as open to inspection under Sect. 91 comprises also the use of acids of structure R1-(CH2)x-R-(CH2)x-R1, where a is a whole number greater than 1 and preferably from 2 to 6. This subject-matter does not appear in the Specification as accepted.