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The main selling point of the Phillips screw in the 1930s was the recessed head, self-centering screw. Straight-bladed screwdrivers tended to slip out of screws slots, ruining wood or other material, occasionally even injuring the worker. The recessed Phillips screws allowed a closer and tighter fit than the conventional slots, and were less likely to slip.
From David Feldman, author of Why Do Clocks Run Clockwise
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