Originally Posted by Jonny B

A 27" frame can definately take the smaller 700cs, going up to 27" on a 700c frame would depend on the frame (close-clearance racing frames - probably not; relaxed touring/hybrid frames - possibly). But why would you want to? I'm assuming you want to put 700cs on a 27" frame?

I think 700c refers to the nominal tyre diameter of 700mm (650c has 650mm tyre diameter). I'm not sure what the letter refers to (there's also 700b).