The bulk of a zip consists of two strips of fabric tape, one permanently fixed to each of the two flaps to be joined, and each carrying tens or hundreds of specially shaped metal or plastic teeth. Another part, the slider, which is operated by hand, rides up and down the two sets of teeth. Inside the slider is a Y-shaped channel that pushes the opposing sets of teeth together or apart, depending on the slider's direction of travel. The zip is usually fitted vertically in clothing so that pushing the slider from top to bottom opens it, and pushing it from bottom to top closes it. The friction of the slider against the teeth causes a characteristic buzzing noise which is probably the origin of the name "zip".