Also known as a quartz lock.

A control on a DJs turntable which causes the pitch control to be bypassed and forces the deck to spin at exactly the base speed ( 45 or 33 rpm). Theoretically, you should be able to get the same effect by setting the pitch control to 0%, however if the pitch control isn't adjusted perfectly the actual speed might be slightly slower or faster than the base speed. This isn't usually a problem when beatmatching since everything is relative, however sometimes ( such as when you are trying to work out the bpm of a track) it is useful to know that the current track is playing at exactly 33 or 45 rpm. This is what the crystal lock is for.

On the SL 1200 deck the crystal lock is situated in the middle of the pitch control slider. If you set the pitch control to exactly 0% a little green light will come on indicating that the crystal lock is now engaged. This can be a problem if your pitch control isn't adjusted perfectly as described above, since you may find for example that when you move the pitch control from -0.25 % to 0% the track actually slows down when the crystal lock engages. For this reason DJs try to avoid setting the pitch control to exactly 0% while beatmatching. Some other turntable models avoid this inconvenience by using completely separate switch for the crystal lock.