Replacing mountain bike bars & stem with drop bars I have a mid-1990s Specialized Rockhopper. I've trashed the flat bars that were on it previously and I want to install drop bars, but to make that comfortable for me I'll want a shorter and higher stem.

So I need to purchase:


A short riser stem that will fit the mountain bike's nonthreaded headset correctly 
A set of drop handlebars that will fit the stem's clamp


but I am lost in measurement purgatory. I've measured the diameter of the tube to which the stem will clamp (with a measuring tape): outside is 28mm and inside is ~25mm. Ok, great! Let's go check out some stems...



...uh.

So the last measurement (80/90/100/110 mm etc) is probably the length of the stem, and the degree is the amount of its rise. What on earth are all of the other numbers? What types of stems usually fit this kind of bike? Are there drop handlebars that usually fit this kind of stem, or do those tend to be a different diameter (because they usually go on road bikes)? 

What on earth are the measurements I need to be shopping with?
There are four measurements you need to be concerned with. The two most critical are the fork steerer diameter and the bar clamp diameter. 

The fork steerer diameter is normally referred to as 1", 1-1/8", or in some cases 1-1/2". The bar clamp diameter can be 1"/25.4mm (typically MTB), 26mm (typically road), 1-1/8"/31.8mm (commonly called oversized or OS). 

The other dimensions are the length of the stem (normally the distance between center of steerer clamp and the center of the bar clamp) and the angle, which is usually stated as degrees from perpendicular. Most stems can be mounted pointed up or down but check the installation sheet first. If two stems are 120mm long but one is +7 degrees and the other is +34 degrees, the 34 degree stem will make the bars feel higher and closer.