See Seatpost Database
At MEC.ca website, you can see a typical seat post called MEC 350mm Alloy Seat Post. 350mm is the length. That model comes with the following diameters to fit different tube sizes: 25.4, 26.8, 27.0, 27.2, 30.0, 30.9
1. Two bolt system (2 vertical bolts)
1. Rocker clamp (one vertical bolt)
3. Single cross bolt with toothed washer
It's difficult to explain the different types of clamp if you haven't seen them. So I refer you to a couple of high quality videos. In the videos, pay attention to the method of adjusting the seat angle.