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A crystal, in electronics, is a device that creates a clock signal at a fixed and stable frequency. To do that, it relies on a fundamental property of some crystals, which is that they change shape very slightly when in an electric field. A voltage applied across the crystal causes a very slight change in its shape; when that voltage is eliminated, a very small secondary voltage is produced as it regains its original shape. In an appropriately arranged circuit, this can be used to cause the crystal to oscillate at a fixed frequency. The exact frequency is set by the physical attributes of the crystal; it is therefore possible to 'tune' a crystal to a particular target frequency by physically adjusting the crystal, e.g. by delicately grinding it.