I just had a debate with my co worker about frame size. As in when someone says "i am going to buy a size 56cm or size 54cm. They are refering to the TT top tube right? Obviously the size number and tt number does not match exactly, most companies are off by a few numbers. as in a 54 will be 54.4 or something like that.

In short, when one refers to a size spec frame 52, 54, 56, where does this number come from? From my knowledge it main refers to the top tube, i maybe wrong, please correct me. My co worker says that it refers to the seat tube....