A free movement, arris protection, construction joint has a pair of arris protection members formed complementarily from strips of sheet with a continuous trapezium wave form. A divider one of the members is typically 100 mm deep for a nominal 0.1 m deep slab. The other one is typically 50 mm deep. The members are of steel plate.The wave form is comprised of flanges extending in the length of the joint and of webs angled to the flanges and the length of the joint. The flanges are spaced on opposite sides of a mid-plane of the joint. The members are bolted together with flangible nylon bolts, with their top (in use) edges flush. Welded to the outer ones of the flanges are L strips having apertures in their flats extending from the flanges for anchoring the joint to its slabs.