Teeth that interlock
Sundback increased the number of teeth and designed them so that the slider locks the teeth from both sides into each other. He also invented a machine for making zips. They were originally used on shoes and tobacco pouches.
Why YKK
On most zips you will find the letters YKK. The Japanese company makes a huge share of the world's zips, from cheap ones to special waterproof ones.
Tip
When a zip keeps coming apart, it is often enough to gently squeeze the slider with pliers. The teeth then join together again.