Ducts are used in heating, ventilation, and air conditioning (HVAC) to carry conditioned air from place to place.

In the past, ducts were commonly made of galvanized steel in rectangular, round, and oval shapes. Various fittings allow transitioning between the various shapes and creating "tee" connections where the air flow can be divided into two or more downstream branches. Many factory-made shapes and sizes are available but galvanized steel can easily be cut and bent to form additional shapes when required. Steel ducts are commonly wrapped in fiberglass thermal insulation, both to prevent the conditioned air from gaining or losing heat through the duct walls and to prevent water vapor from condensing on the exterior of the duct when the duct is carrying cooled air.

Nowadays, while galvanized steel is still very common, many rectangular ducts are manufactured from duct board, a rigid form of fiberglass that can easily be field-fabricated to make custom shapes and sizes. The fiberglass provides built-in thermal insulation and the interior surface absorbs sound, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed knife along the board; the knife automatically trims out a "valley" with 45° sides; the valley does not quite penetrate the entire depth of the duct board, providing a thin section that acts as a hinge. The duct board can then be folded along the valleys to produce 90° folds, making the rectangular duct shape in the fabricator's desired size. The duct is then closed with duct tape or aluminum tape.