The Square Head Screwdriver Bits are made of S2 alloy steel. Dimensions: 1 x S1: 2.4mm - 2 in 1 x S2: 3.0mm - 2 in 1 x S3: 3.4mm - 2 in. Once a rare tool, the Square Head is now commonly used in many products including precision electronics, small household appliances, furniture assembly. Made of best quality S2 steel for best durability and low wear. Works with all standard .25 inch screwdriver and drill chucks including pneumatic, electric, rechargeable. Made of best quality S2 steel for best durability and low wear. Contains 1 of each head size #1, #2, #3 3 bits total.