A folding door is a type of door which opens by folding back in sections or so-called panels. Folding doors are also known as 'bi-fold doors', in spite of them most often having more than two panels. Another term is 'concertina' doors. Folding patio doors are used to expand home interiors for seamless indoor-outdoor transitions. Often referred to as bi-fold doors, accordion doors or folding glass doors, these systems fold in on themselves, creating larger, more unobstructed openings than most sliding patio doors.