Ok I think I have to opt out CVS then.

Onto VSS,
When we checkin one file from VSS, does it lock that file so that no one lese able to do chages it to it???
Or it just merge with the current file at VSS DB when we check out?
My point is ,say can two ppl do changes to same file , at the same time and then checkout to have both changes ?
This is the doubt I have.