On MS Windows, US “qwerty” keyboard:
,(<) = 188
.(>) = 190
`(~) = 192
I do not get a response for clicking backtick. I only get 192 when SHIFT is pressed for a tilde.
EDIT: mea culpa, I had a shortcut assigned to backtick key.
BTW, keycodes can differ between Windows and Mac keyboards. This topic has been discussed over the years. If you cannot find it here, check the Ruby forum at SketchUcation for topics. (I seem to remember some code snippets were posted that defined VK_ prefixed constants for extensions.)