I've made a list here of the tips I used when looking for an alternative yarn and the fibre and construction characteristics that you should be aware of.
- Choose a yarn of the same fibre, yarn construction and gauge. Hint, it's not unusual for the exact same yarn to be distributed under a variety of labels.
- Compare the weight in relationship to yardage for equal substitutions. Read here for more detail.
- Choose a yarn which has similar memory or drape characteristics if you are changing fibres.
- Choose a different fibre or blend.
- Choose Superwash instead of hand wash only yarns.
- Choose a hand-dye tonal instead of a solid.
- Choose a yarn with halo vs. a yarn with a hard twist.
- Wool has memory and will retain shape.
- Wool varies in it's memory based on the specific breed. Some have bounce and elasticity others are firmer and create a stiffer fabric.
- Superwash yarns have more drape, they won't felt and they have less memory.
- Alpaca is a warmer yarn, it has more drape than wool and some halo.
- Cashmere is soft, light and warm. It does not wear well.
- Angoras are fuzzy and warm. They shed and can mat where there is abrasion.
- Mohair is strong and has luster but has less memory than wool.
- Cotton hemp and linen drape well but have little memory.
- Silk and silky looking yarns have shine and drape but they lack memory.
- Nylon adds strength and durability.
- Blended yarns often mix the characteristics of each fibre, look at the percentages to assess which fibre will dominate.
- Single ply yarns have a softer look and will felt easily.
- More plies that are tightly spun will wear better and showcase stitch patterns to their best advantage.
- Solid colours show patterning.
- Multi-coloured or variegated yarns such as hand dyed, heathers or tweeds obscure or soften stitch patterns.
- Novelty yarns require simple stitch formats.
Keep in mind when you work the swatch for the substitution that small swatches are unlikely to truly represent the nature of a specific yarn over a much larger piece of knitting. Either make a larger swatch or be ready to reassess once you have a larger part of your project underway.