Zippers are always measured from “component to component” (end stops), regardless of zipper style.
Close-end zippers are non-separating and are typically opened and closed with a slider. The bottom stop is built up of a single part and doesn’t allow the whole separation of the chain. These zippers are managed on trousers, jeans, bags, boots, etc.
Open-end (separating) zippers have separated ends. The closing part is joined by a box and pin mechanism implemented on the lower end of the zipper. The zippers are closed using sliders, and are commonly used on jackets and other outerwear.
Two-way separating zippers have separated ends as well. The bottom slider allows movement from the bottom of the zipper. These zippers are used in rainwear, sportswear, and sleeping bag.
Two-Way head to head zippers has two sliders at the center of the chain when the zippers are closed. Head to head zippers can be open-end by pulling the sliders towards the stops, but cannot be separated because the ending parts have two stops that cannot be divided. These zippers are mainly used for bags, backpacks, and luggage.
Two-way tail to tail (back to back) zippers has sliders on opposite ends when the zipper is closed. These zippers can be opened by pulling the sliders towards each other, but cannot be separated. They are used for overalls and anything else.
Length Tolerance in Zippers: Generally zippers can vary when bulk is received due to various factors. Based on JIS-S3015 following tolerance is acceptable in zipper lengths.