A Y-channel and a wedge
What Actually Joins a Zip?
The slider is two simple machines: a Y-channel that hooks teeth together and a wedge that prises them apart.
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Step 1 of 5
The teeth are hooks, not a straight edge.
Each tooth has a bump on top and a dimple beneath, so a closed zip is really every tooth locked into the two teeth opposite it.
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Step 2 of 5
Closing is a Y-shaped channel.
Two separate rows enter the wide top of the slider and are guided steadily inward until they are pressed together and interlock at the narrow end.
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Step 3 of 5
Opening uses a wedge instead.
Running the slider the other way drives a small blade between the two rows, prising each pair of teeth apart in turn as it passes.
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Step 4 of 5
Both directions are the same machine.
Nothing inside the slider changes. Which of its two shapes does the work depends entirely on which way you happen to be pulling.
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Step 5 of 5
A single bent tooth ruins the whole zip.
A tooth that will not seat leaves a gap, and because each tooth relies on its neighbours, everything past that point fails to hold too.
A jacket zip has around 100 teeth, and every one of them has to hook correctly for the zip to hold at all.
The short version
Nothing inside the slider ever changes — which of its two shapes does the work depends only on which way you pull. And because each tooth locks into the two opposite it and leans on its neighbours, one bent tooth ruins everything past it.
Try it yourself
Open a zip slowly and look inside the slider. You can see the little wedge pushing down between the two rows of teeth.