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....

I always thought it referred to the seat tube. Note that some frames are measured center-center, and others center-top. If you're wondering about a specific frame just look at the geomety chart.

From some searching around, I guess my co worker and I are both wrong in a way.

Taken from wiki, "Frame size was traditionally measured from the center of the bottom bracket to the center of the top tube."

However, my question is what does the size 54 or 56cm refer to because neither of the seat tube and top tube ever measure exact 54 or 56cm on modern frames.

Your coworker is less wrong than you . The measurement is typically the length of the seat tube from the center of the bottom bracket to the center of the top tube, or in the case of a compact geometry to the center of the virtual top tube.

This.

Ding, ding, we have a weiner!!!

In the "old" days before compact geometry it went by the actual seat tube, and generally the seat tube was very close to the same length at the top tube on a standard off the shelf frameset. Now a days most people look for the size of top tube that they like. For example I prefer a 55.5 to 56cm top tube so I look for a frame in this rage and then I look over some of the other measurements to make sure it is the right frame for me.

Frame size is an imaginary number. Don't get hung up on it. I've been able to get essentially the same position on bikes with "sizes" from 56 to 61cm.

thanks