24 inches

of Mount, Seated or serving on horseback or similarly; as, mounted police; mounted infantry., Placed on a suitable support, or fixed in a setting; as, a mounted gun; a mounted map; a mounted gem.

ceiling mounted, inline and wall mounted (external) ventilation fans

The game of rugby mounted on a horse.

Recessed mounted is inside a box with 1 side open, have louver then mounted inside the ceiling facing the opening downward. While surface mounted is directly screwed on the ceiling.