Brass Knuckles are metal devices with holes punched through them, and are thus intended to be worn on the fingers while the fist is clenched. They are meant to increase the force of a punch, while minimizing the initial force needed to throw an arm, and when used properly, they create massive damage against an intended target and can kill with only 2 or 3 direct hits. Brass Knuckles have been available in Grand Theft Auto games since Grand Theft Auto: Vice City, and have been available in every subsequent game except Grand Theft Auto Advance, Grand Theft Auto IV, (and its episodes) and Grand Theft Auto: Chinatown Wars. In each game, the force of a single Brass Knuckle punch is enough to knock a person to the floor.
In some missions when you must enter somewhere unarmed, the knuckles won't be thought by people to be a weapon.