Usually t6 and t8. Sometimes the larger spydies will have larger though (para 2, spyderco manix xl are t10 for example) I have a t6, t8, t9, and a t10 and I have yet to come across a knife that had anything that didn't fit those.
I bought some a sears. If you go in the store you can bring your knives and test to make sure. And sears will replace any that strip or start to wear out free of charge.
Yep tripwire has is right. :thumbup:
Cheers!