Where are the mouse button actions defined?

[Chris G]

I'm not sure that this is an xfce question or an X question but I'm
sure someone here will know the answer.

Where are the mouse button actions defined?  I.e. where is it defined
that a double-click of button 1 will select a word and a single click
of button 2 (middle) will paste it?

Alternatively (and better for what I want to do probably) where are
the button numbers assigned to 'Left', 'Middle' and 'Right'?

-- 
Chris Green