Zip Top

A zip closing the full opening seals the bag along its whole length. Two rows of teeth are drawn together by a slider, so nothing enters or leaves except through the pull. Nothing else in common use shuts a bag completely, which is what makes it the reference point for both security and weather.

Zips are sized by gauge, the width of the closed teeth in millimeters. A number 5 zip, 5 mm across the closed chain, is the handbag standard and survives daily use. Number 3 is lighter and belongs on interior pockets. A number 3 running a main opening tells you the bag was built to a price. Number 8 and above belong on luggage, where far more force pulls the teeth apart. Coil zips, whose teeth are a continuous nylon spiral, recover if they separate. Metal tooth zips are stronger but jam permanently once a tooth bends.

Two hands and a few seconds is the running cost. Add a part that will eventually fail and is expensive to replace once the bag is sewn around it. Run the slider the full length twice before buying. It should not need coaxing at the corners, and it must not catch lining fabric. That catch is the most common cause of a ruined zip. How the zip is set matters as much as its quality. Stitched into a gusset, a fabric panel that lets the opening spread, the mouth opens wide and stays usable. Sewn straight across a narrow top, the bag becomes a letterbox you fish around in.