Air register

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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.

One of the more common diffusers, a round ceiling type. (Titus Manufacturing Corp.)

For heating, the preferred location is in the floor, at the baseboard, or at the low sidewall of the outside wall, preferably under a window. For cooling, the preferred location is high on the inside wall or the ceiling. For year-round air conditioning in homes, a compromise location is the floor, baseboard, or low sidewall at the exposed wall, especially if adequate air velocity in an upward direction is provided at the supply outlet.