http://en.wikipedia.org/wiki/Maidenhead_Locator_System

©Wikipedia ©Wikipedia
Character pairs encode longitude first, and then latitude.
The first pair (“field”) encodes with base 18 and the letters "A" to "R", representing 10° band for latitude, and 20° band for longitude
The second pair (square) encodes with base 10 and the digits "0" to "9”, representing 1° band for latitude, and 2° band for longitude
The third pair (subsquare) encodes with base 24 and the letters "A" to "X", representing 2.5 minutes band for latitude, and 5 minutes band for longitude
The fourth pair (extended square) encodes with base 10 and the digits "0" to "9", representing 0.25 minutes band for latitude, and 0.5 minutes band for longitude
The fifth and subsequent pairs are not formally defined, but recycling the third and fourth pair algorithms is one possible definition:

Sample Code:
BL11bh16
BL11bh16oo66