Behalf

In modern usage, “behalf” is an invariable noun, meaning it has no plural form. In the 15th and 16th century you did see “behalves” but it has fallen out of use, the theory being that the collected persons on whose behalf you are acting are indivisible.

But the lawyer in me thinks - is that so? A lawyer acts not on her clients’ behalf, but on their behalves — and those interests may conflict.