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What do the numbers in yarn sizes mean?

Where do I go to find an explanation of yarn types?  For example what does the 5/2 or the 10/2 etc mean?

In the yarn industry they have a device that looks like a knitting needle gauge.  A ruler with holes in it.  The largest hole is a #1 and I don’t know what the finest hole is, but it is small.  The holes get smaller and smaller as you go along and the typical numbers are like this 1,3,5,8,10,12,14,16,18,20,24, 28, 30,32, …….and then the gaps get bigger 36, 50,60, and then the gaps get much bigger 120, 240 and you are now looking at a yarn finer than the finest hair, who knows how anyone can even see them.  So a 5/2 cotton is a number 5.

The other part of the fraction is the ply, so a 5/2 cotton is a number 5 size and it is a 2 ply.  A 10/2 cotton is exactly twice as fine as a number 5 and it is also a 2 ply yarn.  The number five also gives us more information about the yarn.

Every different type of fibre has a count.   The count for cotton is 840.nn  That means that if you take 1 lb of nice fluffy cotton and spin it to the same diameter as the #1 hole in our thingme the end result will be a piece of cotton yarn that is 840 yds long.  That is cottons count.   You multiply the number of the yarn by 840 and it will tell you how long the yarn is, but then you have to divide it by the ply.   So 5 x 840 = 4200 / by 2 =2100 yds.  There is 2100 yd in a pd of 5/2 cotton.  Lets try that again for 10/2 cotton .  You take the cotton count 840 x # 10= 8400/2 for the ply which gives you 4200 yds. in a pd of 10/2 cotton.  Many different fibres have different counts.  Linen’s is 300.  It is less that cotton because linen is dense and can’t be fluffed up and stretched.    Wool is 540 (I think) and on and on and on.