Separable Fastener — The Zipper

Open US 1,219,881, filed 27 August 1914 and granted 20 March 1917. Sundback's own drawings show the tooth profile and the slider channel more clearly than any modern diagram.

Cut the stops off an old metal zipper and slide the pull right off the end. You now have the three separate parts the patent describes: two toothed stringers and one slider.

Every tooth has a nub on one face and a hollow on the other. That one asymmetry is the whole invention — the nub of each tooth sits in the hollow of the tooth from the opposite side.

Compare teeth from both stringers. They are the same part, not a hook-and-eye pair — which is exactly what Judson's earlier fastener got wrong.

Record the spacing. Sundback's key number was roughly ten teeth per inch — far denser than earlier fasteners, and density is what makes the closure continuous rather than a row of separate clasps.

Sacrifice one slider. File or cut through the outer wall so you can see down into the channel while it is still in one piece.

The wedge dividing the Y-channel is the plough. Trace how the two paths converge into one on the closing side and diverge into two on the opening side.

Nothing inside clamps. The slider only sets the ANGLE at which the two tape edges meet, and the teeth do the locking themselves. This is the point most explanations get wrong.

Cut two large nub-and-hollow teeth from stiff card or scrap sheet and interlock them by hand. Feel how they resist being pulled apart sideways but release freely when swung open.

Try flexing a stringer. The woven tape carries the tension along the row so the teeth stay in register — sew teeth to a stretchy fabric and they pull out of pitch and burst open.

A zipper without a bottom stop simply runs off the end. The stops are not trim — they define the travel and hold the slider captive.

Baste first, then stitch close to the teeth with a zipper foot. Keep both tapes under equal tension — an unevenly sewn zipper drags forever after.

This is the classic failure and it is almost never the teeth. A slider whose walls have spread stops forcing the teeth fully home — squeeze it gently back and it usually closes properly again.

Load a closed zipper sideways across the teeth and then lengthwise along the tape. It is enormously strong across the row and weak in peel — which is why zips fail at the ends first.

The patent and its date. Gideon Sundback filed on 27 August 1914 and was granted US 1,219,881, "Separable Fastener", on 20 March 1917, assigned to the Hookless Fastener Company. Sundback was Swedish-born, trained in Germany, and emigrated to the United States in 1905. The patent is titled for a fastener, not a zipper — the name "Zipper" was coined in 1923 by B. F. Goodrich for a rubber overshoe that used it, and it named the sound, not the mechanism.

Why the earlier attempts failed. Whitcomb Judson patented a "clasp locker" in 1893 and showed it at the Chicago World's Fair, where it drew attention and almost no orders. His design used alternating hooks and eyes — two different parts that had to align perfectly — and it sprang open under load, jammed, and could not be laundered. Sundback's insight was to abandon hooks entirely: make every element IDENTICAL, give it a nub on one face and a hollow on the other, and let it interlock with its opposite number. Identical parts also meant a machine could stamp and clamp them onto tape continuously, which is what made the thing manufacturable rather than merely clever.

The slider is a cam. The Y-shaped channel with its central plough does two jobs with no moving parts: closing, it converges the stringers so each tooth is pressed into the hollow of the one opposite; opening, the plough wedges between the rows and levers them apart. Nothing latches and nothing springs. Because the lock is purely geometric, a zipper's strength across the row is very high — a closed zip resists a surprising sideways load — while its resistance to being peeled apart from one end is low, which is why failures start at the stops.

How it actually spread. It was not fashion. Zips went into military gear in the First World War — flying suits and money belts — then into boots and tobacco pouches. Clothing resisted for two decades, partly because a fastener that could be opened quickly was thought improper. The turn came in 1937, when French couturiers took it up and Esquire called it the "Newest Tailoring Idea for Men", pointedly noting that it avoided "the possibility of unintentional and embarrassing disarray". A twenty-year gap between a working patent and general adoption is normal, not exceptional, and it is worth remembering when judging any new mechanism by how fast it catches on.