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Center-to-Center
From WikiPedalia
In measuring distances between round things, whether they're tubes or holes, the usual method is to measure the distance from the centers of the circles.
In the case of measuring bicycle frames, all measurements are assumed to be center-to-center except for the seat tube length.
Seat tubes are sometimes measured from the center of the bottom bracket, but the upper end point may vary depending on the measurement system chosen. Center-to-center seat tube measurements measure to the intersection of the center of the top tube with the center of the seat tube.
See also the entry on "Frame Size. "
