By Giles Long, C4Paralympics reporter, three-time Paralympic champion
Swimming is one of the few Paralympic sports which allows competitors with varying disabilities to compete against each other.
So swimmers with cerebral palsy, spinal injuries, limb loss from birth or amputation and dwarfism can compete - their classification depends on how much their disability affects their mobility through the water.
Ellie has a high level of drag when she swims but freestyle is her strength because she has good balance and hold of the water. Natalie has to work harder to deliver power evenly through her body because of her disability.
However, as she is taller than Ellie she has an advantage at the start of the race with the dive and turning off the wall. It all tends to even itself out.
The S stands for freestyle, backstroke and butterfly strokes. Breaststroke is classified separately as SB because it requires a greater propulsion than the other three strokes.
So a swimmer who competes in multiple events may find their SB breaststroke classification is different from their S freestyle, backstroke or butterfly classification. The final classification is SM, which is the individual medley.
Swimmers are split into three categories:
1-10 - Denotes swimmers with a physical disability. One is the most severe - a person who requires a motorised wheelchair to move around and the higher the number, the lesser the impact of their disability to perform their stroke.
11-13 - Signifies swimmers with a visual impairment, with 11 denoting blindness or little sight up to 13, where swimmers have limited sight. Visually impaired swimmers utilise a "tapper" - a person who indicates when the end of the pool is coming up during a race by gently poking them just before the turn. Swimmers in the S11 category also swim with blackened goggles to ensure that partially-sighted swimmers can fairly compete against blind swimmers.
14 - Swimmers with an intellectual disability.
Swimmers can start a race in the water or make a diving start, which is taken into consideration in their classification.
GB will have 44 swimmers at the London 2012 Paralympics after two sets of trials - The British Gas Swimming Championships at the Aquatics Centre and then the British International Disability Championships at Ponds Forge in Sheffield in April.
- Watch Giles on Road to London 2012: Paralympics Extra's dedicated swimming programme
- Video of C4 swimming reporter Rachael Latham discuss swimming classification
- For all your latest news ahead of the 2012 Paralympics, follow @C4Paralympics on Twitter and like the C4Paralympics Facebook page