9V

Unlike AA, AAA, C, and D cells, the 9V battery isn't a single electrochemical cell -- it's a small rectangular pack containing six individual 1.5V cells wired in series inside one case, which is why its nominal voltage lands at 9.0V rather than a simple multiple of a single-cell chemistry's native voltage. The format, commonly called PP3, was introduced by Eveready (Ever Ready in the UK) in 1956 and formally standardized by ANSI in 1959.

Its distinctive dual snap terminals -- a smaller male terminal and a larger female terminal on top of the case -- set it apart from every other common consumer battery format's cylindrical or coin-shaped contacts, and prevent the battery from being inserted backwards.

Because a 9V pack is really six small cells in series, its usable capacity (roughly 400-600 mAh depending on chemistry and discharge rate) is much lower than a single D or C cell despite the higher voltage, making it a poor fit for high-current devices. It has instead become closely associated with smoke and carbon monoxide detectors, guitar effects pedals, and multimeters -- devices that draw very little continuous current but benefit from a compact, high-voltage source. Many fire-safety campaigns use the twice-yearly clock-change as a reminder to replace smoke detector batteries.