reversiblewith inside hexagon, 1/4" or 5/16", DIN 3126/ISO 1173 D 6.3 or D8for direct acceptance of bits 1/4" or 5/16" outside hexagon C 6.3 (size 4: 22 teeth, size 5: 30 teeth)internal hex drive with a collar-thrust springbits 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)