In mathematics, a type of algebra concerned with logical objects or relations in which there are only two stable states: yes/no, true/false, on/off, etc. Boolean algebra was devised by English mathematician George Boole in 1847, but was largely ignored until the middle of the twentieth century, when it could be applied to the fields of relay switching and, consequently, electronic computers. In Boolean algebra, variables do not represent numbers, but rather statements and logical operations. By using Boolean operators such as "or," "and," and "nor," computers can be instructed to do certain things. For example, a program may read somthing like, "If A or B = C, then do D." The computer will then wait for the conditions to be such as that A or B = C, and then it will execute whatever D correcponds to.