The stick model (5A/5B/7A/7B/etc.) refers to the length and thickness of the shaft and taper of the stick.
The numerical portion signifies the circumference of the stick. In general, the lower the number the larger circumference, and the greater the number the smaller the circumference. For example, the 7A is smaller in circumference than a 5A which in turn is narrower than the 2B. The exception is the 3S, which is larger in circumference than a 2B despite its number.
The letter suffix: "S," "B," and "A" originally indicated the recommended application.
- "S" model sticks were designed for Street applications such as drum corps and marching band, and are typically the largest diameter sticks.
- "B" model sticks were intended for Band applications such as brass bands and symphonic concert bands. Smaller in circumference than the "S" models, they were easier to control and thus especially popular with beginning drummers. To this day the 2B is recommended by teachers practically everywhere as ideal starter sticks.
- "A" stands for Orchestra. "A" model sticks were designed for big band or dance type orchestras. They're smaller in circumference than "B" series sticks and lend themselves well for softer type playing. Nobody really knows why Ludwig chose to use "A" for "orchestra" (I think it was because they didn't like the way the "O" printed on the stick), but they were the first and now everybody uses it.