Linux programs the first PC’s PIT to issue timer interrupts on the IRQ0 at a (roughly) 100-Hz frequency, that is, once every 10 milliseconds. This time interval is called a tick, and its length in microseconds is stored in the tick variable. The ticks beat time for all activities in the system; in some sense, they are like the ticks sounded by a metronome while a musician is rehearsing.
