In air conditioning systems, ducts are used to carry conditioned air to different zones of a building.
The most common shape for air ducts in buildings is rectangular.
Rectangular ducts are preferred because they can fit into narrow ceiling spaces and are easier to install along walls and partitions.
Round ducts are also used but mainly where space allows and for smaller applications.
Square and elliptical shapes are uncommon for practical duct design.
Therefore, rectangular is the correct standard shape for air conditioning ducts.