Socket screws have an allen wrench drive.

Metric socket button head, 1.5mm, 40mm

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.