A jacket hangs closed on a cool day. Its zipper runs from bottom to top, with metal teeth interlocked in a solid line. The two halves of fabric meet seamlessly, forming a barrier that holds shape and warmth inside.
This zipped state maintains enclosure steadily. The slider sits at the collar, and the chain of teeth resists any pull apart.
The slider begins its path downward along the track. Teeth disengage one section at a time, but the remaining chain stays secure.
The threshold occurs exactly when the slider passes beyond the final tooth at the bottom stop.
Before this instant, the lower edges stay connected under tension. In the next moment, the halves spring apart freely, leaving the front fully open.
The garment crosses from bound together to parted open. Visibility shifts from contained interior to exposed chest and shirt beneath. This marks the boundary passed.