Resistor–transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL). The concept had been used in early computers with electron tubes, and in RTL circuits constructed with discrete components, but in 1961 it became the first digital logic family to be produced as a monolithic integrated circuit. Such integrated circuits were used in the US space program in 1962.