A few days ago, I quickly read something here that certain bikes/frames were properly measured from Center to Center (center of crank, to center of top tube, i think). Then, I saw in the marketplace, someone selling an early 90s Specialized Allez, which is more or less what I have (a '92 or '93 Allez Comp). In there, they state it is some number of cm, CtC measurement for the frame. Should my Allez be measured "CtC"? I just bought it, and the seller stated that it was a 56 cm (he is the original owner), but I measure it conventionally as a 58, measuring it from the crank center to the juncture of the seat post and top of seat tube. Which method would show me how Specialized sized this frame?

Danny

Hard to say. Size is a thing the manufacturer determines and many don't size their bike by the same methods. They only generally are the length of the seat tube on old bikes. And even then some mfrs went to the very top of the seat tube, some to the top of the top tube and others to the center of the top tube and maybe a few other ways.

Sloping top tubes have added a few more ways to figuring out a size too. Now it's common to draw a imaginary horizontal line from the somewhere around the top of the head tube and extend it to the extended center line from the seat tube and measure from that intersection to the BB center. But only some do that and others do something else.

So if you are buying a bike. Just know what measurements you need and don't worry so much if the owner says it's one thing and you think it's another. Especially if you are only 2 cm off.

Center to center frames can be drawn out as a single line, the tube's center line .
then the builder cuts the miters, lengths dependent on the diameters being joined .
the angles taken off a protractor laid on the lines..

For a diamond frame, the relationship between CTC and CTT is pretty constant. For English and Italian frames with standard OD tubes, 56 CM CTC is 57+ CM CTT, 56 CM CTT is 54+ CTC, etc. The numbers may be different for French or oversized tubes - you can find the diameters and o the arithmetic if it's relevant to you.