The timer that is used by the system on x86 PCs is Channel 0, and its clock ticks at a theoretical value of 1193181.8181... Hz, i.e. one third of the NTSC color subcarrier frequency, which comes from dividing the system clock (14.31818 MHz) by 12. This is a holdover of the very first CGA PCs – they derived all necessary frequencies from a single quartz crystal, and to make TV output possible, this oscillator had to run at a multiple of the NTSC color subcarrier frequency.