These are the gaps left either side of the gate between the gate itself and the posts or wall plates. It allows for the swing of the gate as it opens and also for expansion and contraction of the timber. With pedestrian gates allow 20-30mm overall , with pairs allow 40-50mm overall, depending on whether hardwood or softwood. Movement is smaller with wall plates on brickwork, greater with posts, Softwood will expand and contract more than hardwood. Gaps left either side for opening tolerances will be covered by the gate stops.