When a door creates space.
1. Sometimes it’s a question of centimetres (which are lacking). The perfect door needs to be able to make itself small and not occupy more space that you are prepared to give it;
2. ...or perhaps the door can’t be a standard shape, but must be trapezoidal in order to perfectly fit the surrounding geometry: this can happen when the door is installed in a basement room or under the stairs;
3. Sometimes it has to be narrower than usual: the rooms are small and the space available for the door is limited. The width has to be reduced while the height remains the same;
4. ...on other occasions, it needs to be larger than usual: wider or higher, it's up to you.