The first wheel based chair was developed by the Chinese in 525 AD. This was slightly crude but established the fact that people could in fact propel themselves from point A to B without the use of their legs.
King Phillip of Spain refined the chair design in 1595 using four wheels of the same diameter and a servant to handle the actual propulsion.
And finally, Herbert Everest patented a folding wheelchair resembling the modern configuration we are all familiar with today. This was designed to be easily transported in a car which is an ironic fact considering that most wheelchair owners are using the device as the result of a car accident.
The modern chair actually has six wheels. Two small diameter wheels located in front to assist with steering, two large diameter wheels in the rear to handle the weight of the owner and two outer wheels attached to the rear wheels which enable the operator to propel the main wheels.
Therein lies the problem, as propelling the outer wheels is difficult on a sloped surface and stressful on the wrists of the owner. This situation is exasperated when the chair is headed downhill or on a slippery slope.
The solution is a lever propelled wheelchair (picture attached). A picture is worth a thousand words, so I will leave it up to the viewer to ascertain the list of benefits from this design.