In computability theory and computational complexity theory, a many-one reduction is a reduction which converts instances of one decision problem into instances of a second decision problem. Thus reductions can be used to measure the relative computational difficulty of two problems. It is said that A reduces to B if, in layman's terms, B is harder to solve than A. That is to say, any algorithm that solves B can also be used as part of a (otherwise relatively simple) program that solves A.