1. Why it looks different
A 9-volt battery is a rectangular block with two snap connectors on top, not a cylinder. The connectors only fit a matching clip, so it can’t go into a device built for AA or coin cells.
2. Common codes
The alkaline 9-volt is usually printed 6LR61. The older zinc carbon kind is 6F22. Lithium 9-volts are usually marked L522. Check the code on the old battery or in the manual before you buy.
3. What uses one
Smoke alarms, guitar pedals, multimeters, radios, some doorbells and some toys. Many of them are still designed around a 9-volt, so look in the battery compartment first.
4. Alkaline or lithium
Alkaline is cheaper and fine for devices that draw little power. Lithium lasts longer in devices that use more power and copes better with the cold, at a higher price.
5. Tape the terminals
Both snap connectors sit on the top, so a 9-volt can short against coins, keys or a metal drawer. Tape over them before you store or throw one away.