How a zip fastens

A zip, or zip fastener, is a pair of fabric tapes. Each tape carries a row of teeth. The teeth are shaped so that one side can hook into the other, but only when they meet at a precise angle.

The slider is the piece you pull. Inside it, the channel is narrow at the bottom and wider at the top. As you pull the slider up, it forces the teeth from both sides into that narrow channel, and their hooks lock together.

Two rows of teeth

Pull the slider the other way and the channel opens the teeth again. A small wedge in the slider pushes the two rows apart. That is why the same pull both closes and opens the fastening.

Metal zips use separate teeth clamped onto the tape. Coil zips use a continuous plastic spiral. Both rely on the same idea: the shape holds once the teeth are meshed, and the slider is what meshes them.

Why a zip sticks

The end of a zip has a stop so the slider cannot slide off the tape. A separable zip, the kind on a jacket, also has a pin and a box at the bottom so the two sides can be taken apart completely.

If a tooth is bent or the tape is torn, the rows no longer meet in line and the slider cannot mesh them. The fastening is mechanical. It does not stick because of glue.

Two rows of teeth

A zip is two rows of teeth pulled together by a small wedge. The slider does the joining. A zip, or zip fastener, is a pair of fabric tapes. Each tape carries a row of teeth.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a zip fastens needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

Why a zip sticks

Each tape carries a row of teeth. The teeth are shaped so that one side can hook into the other, but only when they meet at a precise angle.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

The teeth are shaped so that one side can hook into the other, but only when they meet at a precise angle. The slider is the piece you pull.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

The slider is the piece you pull. Inside it, the channel is narrow at the bottom and wider at the top.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

Inside it, the channel is narrow at the bottom and wider at the top. As you pull the slider up, it forces the teeth from both sides into that narrow channel, and their hooks lock together.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

As you pull the slider up, it forces the teeth from both sides into that narrow channel, and their hooks lock together. Pull the slider the other way and the channel opens the teeth again.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

Pull the slider the other way and the channel opens the teeth again. A small wedge in the slider pushes the two rows apart.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

A small wedge in the slider pushes the two rows apart. That is why the same pull both closes and opens the fastening.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

That is why the same pull both closes and opens the fastening. Metal zips use separate teeth clamped onto the tape.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a zip fastens needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.