When substituting yarn, it is important to keep the denier ( the thickness of the yarn) the same to the original. Below is a listing of yarn qualities and their categories. You can judge the size or category of the yarn by the needle size and the "swatch" of 4". Though gauge is not as important in some items, it is good to determine if the item you are knitting will be larger ( using bigger denier yarn and needles) or smaller ( using smaller denier yarn and needles )

Hint:  keep my color cards in a binder, sheet protectors keep the yarns clean. I have order forms, swatches and all the info in one book.

Yarn numbers:

3/9

First number indicates the number of plies.

The second number indicates the denier- size or thickness of each ply.

High numbers indicate fine yarn: 22/25/30

Low numbers indiate heavier yarns: 6/8/10

WPI:

Wraps Per Inch

If you have the yarn this is a great way to find out the weight of your yarn.

Take a pencil

Mark 1 inch intervals down the pencil.

Wrap yarn around the pencil. Make sure yarn is up against each other per wrap, not over lapping.

Count the number or wraps between the inch marks.

Then you have  your WPI.