Reference
Definition
What the word means. This is not a work instruction.
Supply mains often get smaller as air leaves the main through branches. A duct can be 24x12 upstream and 20x10 downstream because some air has already been delivered to rooms. Always check if the size change happens at a transition, branch, or fitting before assuming one label is wrong.
When two sizes appear on the same run, find the transition point and confirm which side of it you are working on.