Allen button head

US Fasteners

Socket button head screw

Socket screws

Screws with an allen wrench drive.
Socket button head sex bolt

Sex bolts & Mating screws

Sex bolts resemble machine screws, but with internal threads, and can be used with either specialized mating screws or standard machine screws. Also known as barrel nuts and Chicago bolts.

Metric Fasteners

Metric socket button head screw

Socket screws

Screws with an allen wrench drive.