All natural fibres, whatever their natural length or source – animal, leaf, stem or seed – are first teased out and spun into very long lengths to even them out and make them useable. These lengths of yarn are then either wound into a skein and used as single ply yarn, or plyed–twisted together–to make stronger and thicker yarn, rope or thread. The ply of a working yarn, rope or thread is determined by how many filaments or spun fibres constitute it–i.e. two-ply yarn is made of 2 lengths twisted together, and so on and so forth. A 4-ply wool describes a yarn made from four strands of wool, and embroidery thread for example, is made of six distinct strands, each of which is made of four ply thread.

Within each genre, the higher the number of strands, the higher the ply rating, the thickness and the strength.

Cotton and wool both start off as fluffly fibres that are spun into yarn. Silk either comes in a single long filament, or a series of shorter filaments. Linen and other vegetal fibres are long-ish, but all of these need to be spin to even them out and to make longer and thicker yarns. Artificial fibres are either spun or extruded as single filament. Each filament becomes a strand to be plyed.

Perhaps we are most familiar with the term in knitting wools – all wool is designated and patterns relate  the ply of the wool with  the size of the needles. We know that two ply is very fine  ( and for most of us best avoided), that four ply is also time consuming but makes very fine and lovely knitwear, and so on. Embroidery wool, and all threads are plied – we can split them down or double them up to gain the thickness that we need.

And when used for ply-wood, it’s describing the same basic principle. Layers of thin wood are interlocked and overlaid, built up to create a stronger, thicker, more resilient, board.

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