A zipper looks like two rows of teeth and a tab. Mechanically it is a merging machine. The two rows are deliberately offset, and a Y-shaped channel inside the slider gathers them, rotates them into alternating positions, and presses each tooth into the gap between two teeth on the other side. Pull the slider the other way and a wedge splits them apart again. Every opening and closing is a small, repeated act of alignment, and the place where alignment is hardest is the bottom.
Zippers and their hardware come out of the trims districts that sit beside Guangdong's garment and bag factories, where a designer chooses a length, a tooth type, a tape colour and a slider from what is already on a shelf. That is why the same garment can appear with different hardware between batches. It is also why understanding a zipper is mostly understanding a few small moulded parts rather than a brand.
The Y-channel does all the work
Inside the slider is a shaped passage: two incoming channels converging into one. As the teeth enter, the channel forces each one to tilt and interleave with the opposing row. The geometry has to be loose enough to let fabric, dust and the occasional stray thread pass, and tight enough that the teeth cannot skip past one another. Those two requirements fight each other, which is why a slider that has been squeezed in a door, or worn at its mouth, starts to let teeth separate above it even though the teeth themselves are intact.
Once the slider has passed, nothing holds the two rows together except the shape of the interlocked teeth and the tension in the tape. So a closed zipper that "opens behind the slider" is almost never a tooth problem. It is a slider whose channel has widened, or a tape that has stretched so the teeth no longer arrive at the correct angle.
Why the bottom is the weak point
On an open-ended zipper, the two halves must be joined before the slider can do anything. A pin on one tape goes into a box on the other, and that tiny moulded pair has to hold the two rows in exactly the right relative position while the slider starts to move. It takes the whole sideways pull of a jacket being tugged closed, at a single small contact area, every single time. At the top, by contrast, the load is spread along rows of interlocked teeth.
So the familiar failures are predictably bottom-shaped: the pin wears or cracks, the box splits at its seam, the tape tears just below the box where it was stitched, or the pin is not pushed fully home and the slider climbs up with the rows one tooth out of register. That last case produces a zipper that closes with a crooked waistband and jams halfway — and it is a user alignment error that looks exactly like a manufacturing defect.
Three teeth, three failure modes
Metal teeth are clamped onto the tape and can often be pressed back into shape with pliers; a single bent tooth is a repairable event. Moulded plastic teeth are injection-formed onto the tape, and if one shears off there is nothing to clamp — the whole length has to be replaced. Coil zippers use a continuous spiral of nylon or polyester filament instead of separate teeth; they flex well and recover from being twisted, but the slider gradually wears the exposed coils, and a worn coil cannot be re-formed.
This is why a bag and a jacket tend to get different zippers. A bag wants a chunky coil or moulded chain that tolerates being bent around corners; a front opening on a coat wants a firm, flat chain that stays straight under sideways tension. Choosing between them is choosing which kind of failure you are willing to accept.
The tab is a lever
The pull tab is not decoration. Its length sets how much leverage you have over a slider whose friction changes with dirt, fabric and angle. A longer tab makes a stiff slider easier to start, and also makes it easier to catch on something and bend the attachment. A tiny tab on a heavy coat is a design that has optimised appearance at the expense of the exact moment the mechanism is hardest to move.
Equally, a slider that suddenly refuses to move is usually not stiff at all — something is in the channel. A lining, a loose thread, or the edge of the tape folds into the mouth of the slider and jams the teeth at the point where they are being gathered. Pulling harder presses the obstruction in further. The correct response is to stop, back the slider a little, and clear the mouth, which is the opposite of the instinct.
How to read a failing zipper
Start at the bottom. Push the pin fully into the box, check that the box has not split, and look at the stitching immediately below it. Then close slowly and watch the two rows meet: if they interlock cleanly and then separate behind the slider, the slider is worn. If the slider jams at one spot, look for fabric in its mouth before blaming the chain. Only then look at individual teeth.
A zipper is a component that is asked to be a removable seam: it must hold like fabric, open on demand, and survive being misaligned by the person wearing it. Almost all of its engineering is concentrated in a Y-shaped passage and one small plastic pin. The long rows of teeth are the easy part — they just have to be in the right place when the slider arrives.