Reference
Definition
What the word means. This is not a work instruction.
Rectangular duct is usually written width by height, like 24x12. Round duct is usually written by diameter, like 12-inch round. Always confirm whether the size is inside dimension, outside dimension, lined duct, or special material.
On prints, width x height often follows how the duct is shown on plan, but shop standards can vary. Ask when the orientation matters.