How Do Two Rows of Teeth Lock Together? A Wedge and a Y-Shaped Channel

Each element on one tape has a projection on its upper face and a hollow on its lower face, and the two rows are offset so that the projection of one drops into the hollow of the one opposite. The slider contains a Y-shaped channel with a wedge in the middle. Moving it one way forces the two tapes apart around the wedge and lifts each element clear of its neighbour, which separates them. Moving it the other way brings the tapes together at an angle that presents each element to the gap between two on the opposite side, pressing them into engagement in sequence. The mechanism therefore does no locking itself, and the shape of the elements holds them once engaged.

Several features combine to resist forces that would pull it open:

• •Pulling the tapes sideways presses the projections further into their hollows
• •Each element is held by its neighbours above and below as well as opposite
• •Load is distributed along the whole engaged length rather than at one point
• •The tape is woven to resist stretching along its length
• •A stop at each end prevents the slider leaving the tapes
• •Many designs include a catch holding the slider against creeping open

The development ran from 1851 to 1917 and involved several failures. Elias Howe patented an automatic continuous closure in 1851 and never pursued it. Whitcomb Judson exhibited a hook and eye arrangement at the Chicago exposition of 1893, which worked poorly, jammed constantly and sold almost nothing. Gideon Sundback, an engineer working for the company formed to exploit Judson's patents, redesigned it completely, increasing the number of elements per unit length, giving each a projection and a hollow, and designing the machinery to make them, which produced a working device patented in 1917. The name now used was coined in 1923 by a boot manufacturer as a trade name describing the sound.

Failures follow a small number of patterns and most are repairable. Fabric caught in the slider is the commonest, and forcing it worsens the damage, while easing the material out with a pin and working the slider slowly usually clears it. Teeth bent out of line stop the slider engaging them, and straightening one with pliers frequently restores the whole thing. A slider that has been forced or worn opens too wide at the back and no longer presses the elements together, which shows as the fastener opening behind the slider as it moves, and squeezing the slider very slightly closed with pliers is the standard repair. A missing element cannot be repaired and requires the whole tape replaced. Rubbing the teeth with graphite or wax cures stiffness.

The device was reliable from 1917 and did not appear widely in clothing for two decades, which is an instructive delay. Early use was in boots, tobacco pouches and money belts, where the fastening is functional and unseen. Clothing manufacturers resisted because the device cost more than buttons, because it could not be sewn by existing machinery without retooling, and because it was regarded as inelegant. The change came through children's clothing in the 1930s, promoted on the grounds that a child could manage one unaided, and through a decision by French fashion designers to use them in 1937, after which adoption was rapid. The application in the front of men's trousers, which is now the most familiar use, dates from that period.

A Y-shaped channel with a central wedge separates the tapes and lifts elements clear one way, and presents them for engagement the other, while the shape of each element does the holding. Pulling sideways presses each projection further into its hollow. Howe patented a version in 1851 and Judson exhibited a poor one in 1893, and Sundback's redesign of 1917 is what works. Clothing adopted it only from the late 1930s.