Originally Posted by
genejockey
Head tube length and seat stay/seat tube angle.
Bikes sizes are traditionally measured “from” the center of the bottom bracket. The “where” has differed from center of top tube or top of top tube; with some using top of seat tube.
I use the top and downtube attachment distance at the headtube.
An overly simplistic explanation for a vintage bike that has a horizontal top tube, is, as the size gets smaller, the top tube has to be closer to the bottom bracket and by default that brings it closer to the downtube and closer together at the head tube. The head tube generally gets shorter as sizes get smaller.
It is a bit tricky trying to figure a 54cm and 56cm, or 60cm and 62cm from just a picture, but it becomes more obvious when the sizes are further apart. 50cm vs. 60cm is a slam dunk.
John