A ply is a layer of material which has been combined with other layers in order to provide strength. The number of layers is indicated by prefixing a number, for example 4-ply, indicating material composed of 4 layers.
Etymology The word "ply" derives from the French verb plier,Collins Dictionary of the English Language, 2nd Edition, London, 1986, p.1181 "to fold", from the Latin verb plico, from the ancient Greek verb πλέκω.Cassell's Latin Dictionary, Marchant, J.R.V, & Charles, Joseph F., (Eds.), Revised Edition, 1928, p.421
Examples Yarn, where plying is a spinning technique to combine several fibres. Vehicle tires Plywood Toilet paper
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