The bicycle stem is an amazing piece of engineering, if you really stop to think about it. Not only does it have to withstand extreme sudden acute forces applied to it from multiple directions, it must also be finely manufacturered so that there is no slippage at its connection points on your bike, the steerer tube and the handlebars. And all that is accomplished by bike parts engineers in the least weight possible, of course - it always comes down to saving grams when you're building out a high end bike. Like yours! For the reasons just mentioned, bike stems were traditionally made from steel or aluminum, since the bending and breaking points are well understood, fatigue ratings for the metals are well known, and steel and aluminum have proven themselves worthy of the job for so long. Of course, along came carbon, and though there was some early trepidation as to whether carbon could carry out all the things demanded of it - strength, durability, and precision crafting - carbon is now a commonly used material for bike stems. Here, only stems that work with threadless headsets are being discussed - the last generation of stems were called quill stems, and they inserted inside a steerer tube and were secured into place by expansion bolts. The stems in play here are just one straight piece, as shown to the right. It should be mentioned that there was a bried trend during the titanium craze of making TI stems, and Nitto was one notable maker of such exotic titanium bike stems. The bolts which secure the stem to the steering tube can either be oriented so that the heads face the same side, or they can be diametrically opposed. There is some engineering argument as to the relative strength and security by each configuration. The stem plate for the handlebar attachment usually contains 2 or 4 bolts. It seems that there is the occasional discussion about whether or not 2 bolt patterns suffice to hold handlebars in place, so it's worth considering to get a 4 bolt pattern for that extra peace of mind in knowing that your handlebar is that much more secure. Bike stems come in different rises, usually 0, 6/7, 10, 15/16, and 25 degrees. In the case of, say, 7 degrees, it will be written as +/-7 degrees, which means you can orient the stem so that it's pointing up, which is +7 degrees, or by flipping the stem over, you can orient it down, which is -7 degrees. The main measurement in play is the stem length, which is usually expressed in millimeters. Nominally speaking, the usual length stem you'll find on a spec bike is 120mm in length. About the shortest stem you can find is 80mm - remember, the stem is part of a pivoting system with the steerer tube and bike forks, so you have to have some distance on which to pivot. For the same reason, a +35 degree angle stem wouldn't be sensible, since again the stem/steerer tube mating would result in a squirrely steering configuration. Stems can be found as long as 160mm. Generally, you want your bike stem to be in the 100mm to 140mm length range, as a rule of thumb.