A bi-fold gate is two leaves hinged together along their meeting stile, with the outer leaf hinged to a post and the inner leaf supported by a guide roller. As the gate opens, the leaves fold against each other like the panels of a concertina door, halving the effective swing arc.
Bi-folds are uncommon in residential work but useful where the opening is wide (4 to 6 m), the front-clearance is short (a parked car or planter limits the swing), and a sliding gate isn't an option (no runout space along the fence line).
Mechanically more complex than a single swing or sliding gate. The hinge between the two leaves takes a lot of load and needs sturdy hardware (often a heavy butt or pivot hinge with a lockable centre pin). Drive automation for bi-folds is more specialised than for plain swings or slides.