Counter instruments in the Universal and Multi-Function Counters category can include a number if applications. These can include serial events, repetitive events and general frequency events. Since frequency is defined as the number of events of a particular sort occurring in a set period of time, it is generally straightforward to measure. Most frequency counters work by using a counter which accumulates the number of events occurring within a specific period of time. After a preset period the value in the counter is transferred to a display and the counter is reset to zero. The internal oscillator which enables the frequency counter to measure time is called the timebase, and must be calibrated very accurately.