Socket screws have an allen wrench drive.

Metric socket button head

Socket button head screw

Socket button head

Button heads are rounded and low profile. Allen (hex socket) drive is a six-sided recess for use with an Allen wrench (hex key).

See also

Set screw

Metric set screws

Screws with no head and machine threads for use in a tapped hole.

Shoulder bolt

Metric shoulder bolts

Screws with an unthreaded shoulder that is larger in diameter than the threads. Also known as stripper bolts.