What size zipper the pattern uses to draft.

This determines how wide the zipper panel will be by inserting the zipper size into the panel.

The sizes are based off of the brand YKK’s zippers and refers to the width of the zipper coils.

Size guide for Hortensia:

OptionSize in Millimetres (mm)

If in doubt, or if the size of your zipper is unavailable use the size that is slightly wider. This means you will have a wider zipper panel than needed but it is better than one that is not wide enough.