Bi-fold doors, also known as folding doors, come with the ability to fold both in and out in order to open up an area. These doors can be used in both the exterior and interior of the house or building. Mostly exterior Bi-fold doors are used to connect the inside portion of the house to the outside such as courtyards, patios and more. When it comes to interior Bi-fold doors, they are used for the smaller areas of the home.