12"x 8" (Duct Opening Size) 2-Way Stamped Face Steel Ceiling/sidewall Air Supply Register - Vent Cover - Actual Outside Dimension 13.75" X 9.75" it refers to the size

it refers to the size of the "tongue"  so that it will fit in a 8"x8" hole

15" long heavy duty steel construction with matte black powder coating finish

75" with screw holes distance between 12

this ingenious feature prevents the loss of jewelry or small personal items that might otherwise slip through the vents and vanish into the ductwork

The Listed size 24"x20" is for inside duct opening measurement

12"x 8" (Duct Opening Size) 2-Way Stamped Face Steel Ceiling/sidewall Air Supply Register - Vent Cover - Actual Outside Dimension 13.75" X 9.75" it refers to the sizeDurable Construction: This air supply register is made of sturdy stamped steel for long lasting use and reliable performance. Versatile Application: Designed for ceiling or sidewall installation, this vent cover can be used for air supply in various residential and commercial settings. Efficient Airflow: With a 12" x 8" duct opening size, this register allows for optimal airflow distribution from your HVAC system. Easy Installation: The actual outside