 TMs and most modern computers are known as (classical) digital computers, that is, computers that manipulate strings of discrete, unambiguously distinguishable states. Digital computers are sometimes contrasted with analog computers, which manipulate variables that can be continuous. Continuous variables are variables that can change their value continuously over time while taking any value within a certain interval. Analog computers are used primarily to solve certain systems of differential equations (Pour-El 1974, Rubel 1993).