A watch is a small portable clock that displays the time and sometimes the day, date, month and year. In modern times they are usually worn on the wrist with a watch-strap (made of e.g. leather (often synthetic), metal, or nylon), although before the 20th century most were pocket watches, which had covers and were carried separately, often in a pocket, and hooked to a watch chain. Current watches are often digital watches, using a piezoelectric crystal, usually quartz, as an oscillator (see quartz clock). Mechanical timepieces are still used, usually powered by a spring wound regularly by the user. The invention of "Automatic" or "Self-Winding" watches allowed for a constant winding without special action from the wearer: it works by an eccentric weight, called a winding rotor, that rotates to the movement of the wearer's body. The back-and-forth motion of the winding rotor couples to a ratchet to automatically wind the watch. Watches may be collectible; they are often made of precious metals, and can be considered an article of jewelry.