Originally Posted by
svtmike
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.
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.