They are commonly seen on ships, in the military, and in hostels, dormitories, summer camps, children’s bedrooms, and prisons. A ladder or stairs is used to get to the upper bed, which is normally surrounded by a railing to prevent the sleeper from falling out.
Low loft beds are lower to the ground and designed for younger children.
The most common type is the standard bunk bed which has two same size mattresses stacked one directly over the other.
Futon bunks can be used to save space in small apartments or rooms, because the lower bed converts to a couch for use during the daytime.  This makes loft beds an efficient use of small spaces by utilizing the entire vertical area that would otherwise be left unused.
A triple loft bed is an arrangement involving a total of three bunks.