Frame size question
 

I have a 61cm frame but when I look at the geometry chart for my bike, none of the measurements are 61cm. So, what does the frame size actually measure?

A lot of times these days they use virtual points like "effective top tube" which would be the distance from the center of the head tube to the center of the seat post on a horizontal line. It could also be a measurement of the seatpost, from the bottom bracket to that same virtual point

It measures what the length of the seat tube would be

from the bottom bracket center to the top of the seat tube (usually)

if the seat tube were level, as they used to be.

Virtual seat tube measurements are always center to center in my experience.

It depends. All manufacturers size differently. Some go off actual top tube or actual seat tube lengths. Others go off virtual top tubes.

Some stick with historic sizing, so they used to have a 61 and the new geometry is different but they still call it the 61 because that's what they always had there, and that size should fit the people who used to buy the 61.

Others avoid the whole issue with S, M, L.