Originally Posted by
steelbikeguy
The general convention is to measure along the length of the extension. This is illustrated in the photo below by the use of the calipers to measure the extension.
The only exception to this (that I know of) is the Nitto Pearl. In the photo below, there is an illustration on the Pearl's box that shows how it measures the distance perpendicular from the quill to the center of the handlebar.
As shown, the Pearl 9 measures 90mm using Nitto's method, while the conventional method would tend to suggest 100mm.
Steve in Peoria
I suspect early 3TT stems measure differently too as a vendors description measured C to C yielded 90mm but the marking on the stem was 95mm, I don't have other references for this though.