Depends on the carrier and even the market. Anymore you need to know the fare basis to know where it 'ranks'. A decent rule of thumb, the longer the fare basis, the more restrictive and most likely lower it 'ranks'. Also, with the exception of 'Y', the higher up you go in the alphabet, the lower the fares tend to go. This is of course a generalization, but it basically holds true.
"The two most common elements in the universe are Hydrogen and stupidity"