Quill stemsinsert into the threaded steer tube on a bicycle fork and are held in place by means of a bolt which tightens a wedge inside the tube. The other type of stem commonly used is a so-called threadless stem which clamps to a threadless steer tube. Quill stems and threaded steer tubes are less common nowadays, but the Nitto company in Japan still makes a wide range. Here is a side by side photo of most of the stems they make. Each stem in the photo is placed so that the minimum insertion mark on the stem aligns with the top of the ruler. This gives a basic visual cue as to how high the various stems can extend above the top of the tube. Notice that the stems are placed at an angle meant to replicate the 70 some degree angle of the head tube on most bikes.
NEWS FLASH! 09/26/2012
We now stock the 225 Technomic Deluxe stems. These have a long quill like the Technomic but are cold forged and have the same finish as the Deluxe. 25.4 or 26.0 clamp sizes.
The Dynamic stems only come with a 26.0 mm clamp size, and the Dirt Drop, Technomic and Deluxe stems come with either 25.4mm or 26.0 mm clamps. A 25.4 stem will not work with a handlebar that has a diameter at the center clamping point of 26.0. However a 26.0 stem will work with a 25.4 handlebar as long as a shim, specially made for this purpose is used between the stem and the bar. The Dirt Drop stems are only made in 80 mm and 100mm versions (the extension from the vertical-ish riser part or quill part of the stem). The other models come with extensions generally from 80 mm to 100mm in 10 mm increments. Got it?
Hopefully, this is a useful comparison for the bicycle traveler who might be thinking about changing the position of their handlebars or about getting different handlebars.