The power-reserve indicator displays the state of wind of the mainspring. It reminds the owner that the movement must be rewound again before it stops. The power-reserve indicator is a very useful horological complication, as it lets the wearer know whether a watch is wound before putting it on. This is applies particulary to automatics, where the state of the wind is unknown, or to those manually wound watches that may have a power-reserve indicator of up to 8 or 10 days or so. Often called "UP-and-DOWN display" by Lange.