All zips have an even number of identically spaced teeth on both sides. This means that they work well in straight lines, but don't really do curves.

Ellipsa-Zip solves this problem. It comes in the form of a range, which look like segments of an ellipse. You choose the profile and length you want.

The
zip runner can negotiate curves because the teeth on either side are of two different sizes. The inside curve teeth are smaller, while the outside curve teeth are slightly larger and have a wider spacing. The runner is correspondingly uneven, so that it can accommodate and mesh together the larger and the smaller teeth, whose meeting points are designed to fit and lock against each other.

This means that you can now have a customised zip which will snake around any shape, yet fasten and unfasten evenly and smoothly.

If the larger outside-curve teeth have wider spacing, the
little inside-curve teeth won't lock in when they mesh.
Likewise, the inside-curve teeth must be more widely
spaced in order to accomodate the larger outside teeth,
which means both sides will still be the same length and
they won't form a curve. Sorry.

I disagree, and if you make a simple drawing of two sets of teeth running along the edges of two facing curves, you can see that the teeth can meet together along the line of the third curve. The fact that they are spaced and sized differently at their base lines doesn't prevent them meeting and meshing correctly at their point of contact.

I'm with [xenzag]. Imagine a regular zipper, and
gradually expand the teeth on one side. The zipper
will curve away from the expanded side.

However, baked. The bearskins (hats) of the Royal
Horse Artillery have removable liners made of
tweed. These liners are held in place by circular
zippers, manufactured (since 1922) by Zipex using
unequal teeth on the two halves of the zipper.