Internal bifold doors come in two variation. Internal Bifold Door - consist of 2 or up to a maximum of 6 doors depending on the width of your opening. The doors are linked with specialist hardware which allows them to slide and fold together like a concertina, hence they are sometimes called folding doors or concertina doors. Internal Fold Flat Bifold Doors - have between 2 or a maximum of 4 doors depending on the width of your opening. Again these are linked together with specialist hardware but unlike bifold doors that concertina together, Fold flat doors can rotate 180 degrees around against a wall leaving the opening completely free.