CRs:
Bob Hanson's drawing of stem extension measurement http://i32.photobucket.com/
shows a Nitto Technomics stem.
To add further confusion to this stem measurement discussion, I've found a sizing difference between various models of Nitto stems:
I've been using a Nitto Pearl13 stem for years. The Nitto Pearl is measured in the perpendicular dimension. This was shown in a small picture on the side of the blue box in which the stem was shipped. (I discarded the box long ago.) Hence, a Nitto Pearl 13 measures 130mm at 90 degrees (perpendicular to the steering tube), and about 135 mm along the horizontal centerline. My horizontal, along-side-centerline measurement is 137mm (another story, see below.)
I needed to reduce my reach by about 5mm, so I purchased both a Nitto Technomics (tall quill, approx 190mm?) and also a Nitto Technomics SQ (short quill, = 155mm). Both of these stems measure about 130 mm along the centerline of the horizontal part of the stem. These stems are obviously about 5mm shorter than the Pearl 13, which is (at 13 cm) "supposedly" the same size.
I've also found slight differences (about 3mm) in sizing for some of the later-model 3T stems.
(Side note: The Pearl 13 stem angle is approximately 72 degrees. The top part of the stem slopes down instead of being parallel to the top tube of my Richard Sachs bike, which has a 74