Back in the days of the IBM XT, it was 4.77MHZ, then 8 & 12MHz for 286; the 386 was 16, 20, 25, and 33 (plus AMD's 40MHz, though I know there were some other bastard schemes out there with 6 & 10MHz XT, and 10 & 16MHz 286). The Intel camp seemed to settle on 33.333MHz for the 386/486 (and original Pentium?). At some point, the jump to 66.667MHz came along, and then 100 and now 133. How did that come to be? Why 33.333 and the subsequent double, triple and quadruple? How does the timing crystal work?