Horizontal field of view

See also: letterbox

Visual examples
With 4:3 to 16:9 comparison of the effects of FoV types.

Vertical FoV: View is expanded and cropped off from the sides, 4:3 has narrower view horizontally. Ensures vertical vision is the same regardless of aspect ratio.
┌┬───┬┐
││4:3││
└┴───┴┘

Horizontal FoV: View is expanded and cropped off from top and bottom, 16:9 has narrower view vertically. Ensures horizontal vision is the same regardless of aspect ratio.
┌────┐
├────┤
│16:9│
├────┤
└────┘

For example (The Scourge Project, UE3), compare 4:3 resolution:

to 16:10 resolution: