Square key clamp fittings, also called box section connectors or square tube clamps, for building frames from 25 × 25mm and 40 × 40mm square hollow section without welding. Use them for signage frames, display stands, shop fixtures, shelving and industrial furniture, tightened with a single allen key. Square fittings are not for handrails or guardrails; use our round key clamp fittings for anything safety-critical.