 Construction of a quantum theory of a physical system proceeds by first associating the dynamical degrees of freedom with operators. These are mathematical objects on which operations of multiplication and addition are defined, as well as multiplication by real and complex numbers. Another way of saying this is that the set of operators forms an algebra. Typically, it is said that an operator represents an observable, and the result of an experiment on a system is said to yield a value for some observable. Two or more observables are said to be compatible if there is some possible experiment that simultaneously yields values for all of them. Others require mutually exclusive experiments; these are said to be incompatible.