Folding doors are sometimes known as accordion doors, and are a common sight where a long aperture needs to be covered. So a folding patio door folds like a concertina, with the hinge points being guided by a track above and below.

Folding doors tend to be used where opening of a conventional door would be restricted at one or more of the ends, because of space constraints.

In its simples form folding patio doors have two doors, which are known as a bifold door.

Beyond this, it is possible to use either a single long door which folds up as four or more panels, locking at one end: or as folding sliding doors sliding in on the same track on both sides, joining at the centre of the doors.

Unlike sliding doors the door can fold completely flat on closure making it less easy to attack the weakness from outside when two sliding doors run on different tracks so exposing a weakness at the center.

The choice is often one of design: french patio doors, tend to give a more conservative and historic feel, where folding doors, with mutliple panels generally look more modernistic. Used in the home it is important to choose a folding patio door that does not make the house look too much like an office partition.