Zip fastener is a fastening device in which two rows of teeth and sockets are brought together so that they interlock. Metal zips have lines of tiny teeth, while plastic zips contain small loops on each side. When you pull the slide of the zip fastener up, it pushes the teeth or loop together. The first zip was invented by Whitcomb Judson in 1893.
Beneath each tooth in a metal zip fastener is a small socket. The slide is narrow at the bottom so that it forces the teeth together as the zip is pulled up. The teeth on one side fit between the teeth on the other side. As they come together, each tooth slips into the socket under the tooth above and the zip stays closed. As the slide moves down, a divider at the top of the slide pulls the teeth apart.
Top pieces and a bottom piece at the ends of the fastener stop the slide coming off, though some zips are made to separate completely by pulling one line of teeth out of the bottom piece. Plastic zips have two spiral coils instead of lines of teeth but their working principle is the same.
