Register-A device attached to an air-distributing duct for the purpose of discharging air into the space to be heated or cooled. These openings are referred to as registers, diffusers, supply outlets, or grills (see illustration). By common acceptance, a register is an opening provided with means for discharging the air in a confined jet, whereas a diffuser is an outlet which discharges the air in a spreading jet. Both registers and diffusers may be placed at a number of locations in a room, including the floor, baseboard, low on the sidewall, window sill, high on the sidewall, or ceiling.