diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
index 5c17f3f01bec..acaf6328ac49 100644
--- a/lib/matplotlib/lines.py
+++ b/lib/matplotlib/lines.py
@@ -1568,6 +1568,12 @@ def set_xy2(self, x, y):
         """
         Set the *xy2* value of the line.
 
+        .. note::
+
+            You can only set *xy2* if the line was created using the *xy2*
+            parameter. If the line was created using *slope*, please use
+            `~.AxLine.set_slope`.
+
         Parameters
         ----------
         x, y : float
@@ -1583,6 +1589,12 @@ def set_slope(self, slope):
         """
         Set the *slope* value of the line.
 
+        .. note::
+
+            You can only set *slope* if the line was created using the *slope*
+            parameter. If the line was created using *xy2*, please use
+            `~.AxLine.set_xy2`.
+
         Parameters
         ----------
         slope : float