On a faucet with separate hot and cold handles, each handle sits on its own stem or cartridge, and the one on the dripping side is worn. Feel the drip: hot water means the hot side. The hot and cold parts are different part numbers on most brands, so pick your brand before buying.
Pick your brand
MoenDeltaKohlerPfisterAmerican StandardGroheGlacier BayPeerless
Not sure of the brand? See how to find your faucet brand and model. New to this? Start here.