Honestly, I never really thought about the yarn I was substituting. I would find a yarn I loved then pick out a pattern I liked and just begin working up my stitches. I never looked at gauge, I never paid attention the fiber or even the weight of the yarn.
In some cases things worked out fine, in others I found myself with items smaller, or firmer, or just plain awkward looking. So I have learned, and it is time to share some insights.
First realize that the pattern you find was designed in a specific yarn. The way it looks in the photo is because of this specific yarn. Changing the yarn will change the effect, maybe the drape, maybe even the size.
Now what to compare to make the change.
Check the weight of the yarn. The weight is in essence the diameter of the strand of yarn, it can be assigned a number (from 0 lace-7 super chunky) or given a name such as lace, thread, sock, fingering, baby, sport, DK, worsted, Aran, chunky, craft, bulky, roving. These numbers or names are assigned by the manufacture and finding matching yarns at least get you in the ball park that the yarns are similar.
However there are times that you pick up a yarn and it doesn’t have a weight listing by number of name, but it does have a knitting gauge listed. This gauge can help you make the weight comparison too. Yarns that have the same gauge, using the same size needles, will be also be compatible in weight. If the yarns are using the same size needles in the gauge but the stitch and row counts are not the same, the yarn with the higher number of stitches in the gauge will be thinner than the other.
Another way many compare the weight, is to compare the yard/meters and the ounces/grams of the skein. If a skein states that it is 400yrd/366m and 1.75oz/50g it would be compatible with a yarn of 425rds/388m and 1.75oz/50g, but not compatible with a yarn that was listed as 600yd/549m and 1.75oz/50g, as the latter yarn is much thinner. It is a comparison of yards/meters and comparison if ounces/grams that help you see if things are in the same ball park.
The next thing to consider when comparing yarns if the fiber content. In some cases it may not make much of a difference, but a few fibers act completely different from one another. For instance if you are substituting a wool yarn with a lot of bounce or springiness, with a 100% silk you fabric will not even resemble each other. The silk tends to have a lot of drape, it flows, and in comparison to the wool will be limper. Whereas the wool will have some stretch and spring back into place.
Yarns with similar fiber content will behave similarly, so use caution if the labels vary greatly.
Now that you have found a yarn to substitute, if you want to ensure that you will be happy with the outcome of your project, make a gauge swatch. If you make gauge and are happy with how the fabric feels and looks, make your project.