Recent articles on equations characterising the trigonometric functions ([1], [2]) prompt a consideration of still another equation, which might be classified under the heading of “Students’ Mathematical Mythology” In [3], Mr. Heafford multiplies the “obvious” sin ( x + y ) = sin x + sin y by the equally “clear” sin ( x − y ) = sin x − sin y to obtain the (correct!) result, sin ( x + y ) sin( x − y ) = sin 2 x − sin 2 y .