The slider on a zipper acts as both an opener and a closer depending on which way it moves.

The longer version

Inside the slider are two channels that merge into one. Moving one way, it forces the separated rows into a single interlocked chain; moving the other way, its central wedge pries the chain apart again. The same part does both jobs because the geometry is reversible: the wedge that presses the teeth together is the wedge that splits them, travelling backwards.

Nothing inside the slider latches or releases, which is why a zipper can be worked from either end when two sliders are fitted. Friction in the channel is the only thing keeping it where you left it. That also explains the common failure: once the throat of the slider wears wide, the teeth pass through without being pressed together and the chain quietly comes undone behind you.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.