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I purchased the zippers to use in box seat cushions I am making for my sofa. They say size 5 is the one to buy for this type of project. Had to first figure out how to install the zipper pulls. You cut the length you need first, pull the coil open a couple of inches. Then you trim one side of the zipper shorter by about 1/4 inch. I am right-handed so I trim the right side (right side of zipper facing me). Start the curved end of the zipper puller, slide it down onto the teeth about 1/16" down, you will feel it attach. Then push the shorter end up to engage the shorter side into the puller. Pull the zipper down the length and back up, you are ready to install. I find it easier if I place the zipper on the curved arm of an armchair when I start to engage it, if that helps. Youtube has a couple videos on how to attach the pull
Once you do a couple of them, you will totally get it. These nylon ones are a little easier to start than the metal ones. However, they are tougher to sew through. Overall, a must have for making boxed cushions.