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 the 3D Universe since GTA Vice City except in GTA Advance. 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.
In GTA San Andreas, once the player have dated with Katie and Barbara, and they equip the Brass Knuckles, it cannot be discarded and it will be equipped permanently.
In the other 3D games, the Brass Knuckles will remain with the player once the tier 1 cheat has been entered and will remain permanently equipped even if the player enters the tier 2 and 3 cheats unless the player gets wasted or busted.
Strangely, the Brass Knuckles are omitted from Grand Theft Auto: Vice City 10th Anniversary Edition.