The following chart gives some of the letters used to designate each class of service. Note that Fn means Night/Offpeak Coach in the First Class compartment, and Yn means Night/Offpeak Coach in other than the First Class compartment.

In reality there is no difference between classes F and P, nor between classes C and J. The Q class is usually used for discounted student fares. If you want the cheapest fares, look at the Thrift and Coach Economy classes. Airlines have started eliminating First Class, because many companies now have policies that won't let their employees claim a first class ticket on their travel expenses. Instead, the airlines have started upgrading their Business Class as a marketing ploy (and, of course, company policies are now requiring employees to travel coach).