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Old 03-15-2019, 09:29 PM   #10
DDW
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City: San Francisco
Join Date: Apr 2018
Posts: 3,093
Those are shade cord stops. They work because the helical track coming from the center towards both ends is tapered in diameter, getting larger as it goes. The cord catches one of the tracks, is run outboard until the increasing diameter jams the cord. It's quite a clever and reliable design.
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