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.