"Chronometer" is simply a more or less obsolete name for a clock, traditionally a very accurate ship's clock or a railroad clock. The term does live on in Switzerland, where the most accurate mechanical watches may be certified as "Officially Certified Chronometers" by the government. There are a great many different mechanisms used in mechanical clocks, but today's wristwatches all use a variation of the balance wheel escapement. In this system, a small flywheel connected to a light spring rides on hardened pivots made from tiny gemstones, oscillating back and fortth, propelled by a mainspring. As it reverrses direction, a cog attached to the flywheel, or balance wheel, allows a gear (called a "wheel" in clocks) to move a fixed increment, in turn allowing the hands to move a small amount. The balance wheel is adjusted by a system a screws in its perimeter that allow it to be very accurately balanaced, and its mess to be moved inward or outward. IN addition, the tension of the light spring connected to the balance wheel may be adjusted. Between these two adjustments, the period of oscillation of the wheel may be calibrated. While this allows for a very accurate timepiece that may only gain or lose a few seconds a month, it should be noted that this is still several orders of magnitude poorer than can be acheived with electronic timekeepers.