Hoop earrings are circular or semi-circular in design and look very similar to a ring. Hoop earrings generally come in the form of a hoop of metal that can be opened to pass through the ear piercing. They are often constructed of metal tubing, with a thin wire attachment penetrating the ear. The hollow tubing is permanently attached to the wire at the front of the ear, and slips into the tube at the back. The entire device is held together by tension between the wire and the tube. Other hoop designs do not complete the circle, but penetrate through the ear in a post, using the same attachment techniques that apply to stud earrings. A variation is the "continuous hoop earring". In this design, the earring is constructed of a continuous piece of solid metal, which penetrates through the ear and can be rotated almost 360°. One of the ends is permanently attached to a small piece of metallic tubing or a hollow metallic bead. The other end is inserted into the tubing or bead, and is held in place by tension. One special type of hoop earring is the "sleeper earring", a circular wire normally made of gold, with a diameter of approximately one centimeter. "Hinged" sleepers, which were common in Britain in the 1960s and 1970s, comprise two semi-circular gold wires connected via a tiny hinge at one end, and fastened via a small clasp at the other, to form a continuous hoop whose fastening mechanism is effectively invisible to the naked eye. Because their small size makes them unobtrusive and comfortable, and because they are normally otherwise unadorned, "sleepers" are so-called because they were intended to be worn at night to keep a pierced ear from closing, and were often the choice for the first set of earrings immediately following the ear piercing in the decades before ear-piercing guns using studs became commonplace, but are often a fashion choice in themselves because of their attractive simplicity and because they subtly call attention to the fact that the ear is pierced.