There are a number of theories and rules of thumb for substituting one yarn for another:
- If the finished item is to remain the same size, the yarns should be of similar thickness
- Alternatively, you can to change the number of stitches in your pattern (ideally for an even number of pattern repeats) to come up to the same measurements.
- Different fibers will handle differently in wearing and cleaning — cotton knits will become shorter and wider, wool will full or felt, acrylics will pull and pill. Try to keep yarns that will handle similarly after the project is finished.
- Substitutions in both yarn and gauge can be used to scale patterns up and down — use a similar color and texture of fingering yarn to make an American Girl doll's sweater look more identical to that of her owner's worsted sweater, with similar numbers of stitches and rows for each garment piece.
- Most craft yarn manufacturers' yarns of a given weight, fiber blend, and texture can be used interchangeably.