Which Hinge Types are Available?
You will probably come across two types of hinge: flag hinges and butt hinges.
Flag hinges are bigger, and are fixed to the face of the door.
Butt hinges are a lot smaller and discrete, with the fixings being on the side of the door. The way the hinges are fixed to the door is also important.
With a bi-fold of higher quality, the screws that hold the hinge in place won’t screw directly into the frame material, they should screw into either a steel clamp or steel rivnut. The only exception to this are timber bifold doors, where you should be looking for either stainless steel or brass screws.