Swimmers are classified according to how their impairment affects their ability to perform each stroke.

The classification rules of the International Federation for Swimming state that athletes with a physical impairment, visual impairment and intellectual impairment are eligible to compete in this sport at the Paralympics.

Classification also groups athletes in classes, defined by the degree to which they are limited in their ability to perform each stroke. Because of the number of different impairment types that compete in swimming combined with the fact that freestyle, butterfly, backstroke and breaststroke each place different demands on the swimmer, means swimming has a high number of classes.

There are several different classes in Swimming:

1–10: athletes with physical impairments. Class 1 swimmers’ impairment has the greatest impact on their ability to perform strokes; class 10 swimmers’ has the least impact.
11–13: athletes with a visual impairment. Class 11 swimmers have little or no sight; class 13 swimmers have limited sight.
14: athletes with an intellectual impairment compete in class 14.

Breaststroke uses greater leg propulsion than any other stroke, therefore athletes with a physical impairment often have a different class for this event compared to Freestyle, Backstroke and Butterfly. This is also taken into account when athletes compete in the Individual Medley. This is shown by the following prefix:

S before the class represents Freestyle, Backstroke and Butterfly events.
SB before the class represents Breaststroke events.
SM before the class represents Individual Medley events.

So for example an SB5 swimmer will have an impairment that will have a greater impact in the water while swimming breaststroke than an SB7 swimmer. Also that same SB5 swimmer may be an S4, S5, S6 or S7 swimmer for freestyle due to the impact their impairment has on their leg kick in the water.

Read an overview of classification at the London 2012 Paralympic Games