The Paralympics is officially underway, with 12 full packed days of sporting action ahead.
Some of the world's finest athletes have descended on Tokyo to compete in the prestigious competition, which will see 539 events contested across 22 sports.
An important aspect of the Paralympics is the different event classifications, which aim to ensure competition between athletes is fair.
Events are classified by a letter and number combination; the lower the number, the greater the impact an athlete's disability has on their performance.
With GB's golden girl Ellie Simmonds set to race in her fourth Paralympics in the S6 category, here is your guide to the different classifications in the pool.
What do the different letter codes S; SB and SM stand for?
S stands for freestyle, butterfly and backstroke events.
SB stands for breaststroke.
SM refers to individual medley.
For athletes with physical impairments, there are 10 different numeric classifications for each stroke: 1-10.
The lower the number, the more severe the impact of the impairment on swimming.
Visual impairments meanwhile have three numerical classifications, like in para-athletics.
What does S1-S10 and SB1-SB9 mean?
Class S1 is for athletes with a physical impairment competing in freestyle, butterfly or backstroke.
These athletes' disabilities have been deemed to have the most impact on their swimming, while athletes in the S10 category on the other end of the scale are considered to have disabilities which impact their event to a lesser extent.
The same goes for the SB (breaststroke) events.
What do S11-13 and SB11-13 mean?
S11-13 and SB11-13 classify athletes with visual impairments, with the former again referring to freestyle, backstroke and butterfly while the latter refers to breaststroke.
What is S14 and SB14?
Athletes competing in these categories in the pool have an intellectual impairment.
This could cause individuals difficulties regarding pattern sequence, reaction times and memory.
What category does Ellie Simmonds compete in?
Ellie Simmonds competes in the S6 category, racing in both freestyle and individual medley events.
She is the current defending champion in the 200m individual medley SM6, having won the title in both London and Rio.
After racing her first Paralympics at the age of 13 in Beijing, the 26-year-old will look to add to her impressive collection of 8 medals, including 5 golds, in Tokyo.
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