Most screw caps are designed for use with Phillips screws, then adapted for use with square drive. That's fine, except that they just don't stay in the recess. Designed specifically for Square Drive Screws, these have an oversized solid center plug so that it compresses when pushed into the recess, gripping the walls of the recess and resisting removal. Common hollow plug designs conform so easily to the basic recess shape that they easily pop out when bumped or jostled. Fits flat head screws with #1 Square recess (#6's). Caps are approximately 1/2 inch diameter. CAP-06193C is a pack of 100.