Para-swimming classification is a function-based classification system designed to allow for fair competition in disability swimming. The classes are prefixed with "S" for freestyle, butterfly and backstroke events, "SB" for breaststroke and "SM" for individual medley events. Swimmers with physical disabilities are divided into ten classes based on their degree of functional disability: S1, S2, S3, S4, S5, S6, S7, S8, S9 and S10. The lower number indicates a greater degree of impairment. Those with visual impairments are placed in classes S11, S12 and S13. Class S15 is for athletes with hearing loss. Additional classes may be reserved for swimmers with intellectual impairment: S14, S18 for swimmers with Down Syndrome or class-S14 intellectual impairment combined with a physical impairment, and S19 for swimmers with autism.