The vast majority of keyboards manufactured today use rubber domes. These are domes moulded out of silicone rubber, either discretely or (more typically) in form of a single rubber sheet with a dome at each key position. Each dome serves as the return spring, as well as the device which transfers the keystroke force from the operator’s finger onto the membrane assembly.
Rubber domes have the advantage of a rounded tactile peak, compared to the often sharp and jarring peak of many mechanical designs. This affords rubber dome keyboards a softer and more comfortable tactile feel than that of Cherry or Alps mechanical keyboards. The chief disadvantage however is that the behaviour of silicone rubber changes with age, which limits the lifetime of the keyboard to the amount of time until it becomes too difficult to type on. Typically, the rubber domes stiffen with age, requiring ever increasing effort to press keys. More unusually, the rubber can become limp, to the point of creating sensory deprivation from how much it absorbs the interaction force from your fingers.
The exact feel of the keys depends in part on the profile of the domes. In Electronic Engineers Master 1985–86 Volume 2 (page B·1734), Moxness illustrated five different profiles of rubber key, along with the force curve that each one would produce: