A diagram that indicates the possible states of a finite-state automaton and the allowable transitions between such states. There are several different dialects of STDs. Each one depicts the states, transitions, and event(s) that can cause each transition. STDs may also indicate conditions that control whether a legal transition is allowable, or actions that are undertaken either during a transition or on entry to a new state. Because an STD defines a finite-state automaton, the object being modelled may be only in one state at a time. STDs can be used to define the control structure of a software module, or to define the modes of operation of large systems. See also statecharts.