1/4″? inside hexagon, DIN 3126/ISO 1173 D 6.3 Outer 10 mm for use in tools with 10 mm internal hex drive or internal double hexagon (e.g. “FLEXI” double-ended flexi-joint spanners, T-handle universal joint spanners, box spanners, ratchet ring spanners etc.). Internal hex drive with a collar-thrust spring. Bits are easy to insert, lock securely in position and can be removed just as easily;even hex bits with a wide groove (Type E, DIN 3126/ISO 1173). HPQ? high performance steel.