Now that many bikes have slanted top tubes, or no top tube at all, a measurement called "Effective Top Tube Length" is often used. This is the measurement of what the flat (Horizontal) top tube on a bike would be if it were there. To measure the top tube length of a bike, the bike usually has an actual flat top tube and you just use a good old tape measure and get the length of the tube. To measure the effective top tube length (again the bike would not have a flat top tube) you would need to measure from the head tube to the seat post where the flat top tube would be if the bike had one.