Before we look at shortening a zipper, let us look at zippers in general:
Coil Zipper - A more flexible and smoother running than molded teeth zipper. Use #5 for jackets and smaller sizes for other clothing. Use #10 for foul weather gear and heavy jackets. 4.5 or 5 for slip covers
Molded Teeth Zipper - Stronger and more abrasion resistant than coil zipper. However, molded teeth zipper is not as flexible as coil zipper. Use #5 for most clothing and slipcovers. Use #10 for boat canvas, sleeping bags and heavy jackets.
Tooth Size - The distance across the teeth or coils measured in millimeters (mm).
Continuous Zipper Tape - A length of zipper, either coil or tooth style, sold by the foot. Allows you to customize your own zippers. At least one end must be sewn closed. You cannot make a separating zipper with zipper tape.
Separating Zipper - Zippers that allow the two sides to come completely apart and easily place back together. Used in jackets and robes. Larger sizes are used on sailcovers, bimini tops and enclosures. Available in both coil and molded teeth. See diagram below, right.