Medium weight yarn includes the following yarn types: Worsted, Afghan, Aran. Most patterns call for a worsted weight yarn and it really is your basic standard yarn. You can substitute one worsted weight yarn in a pattern for another, just make sure you match the gauge of the original yarn. Knit some test swatches with your substitue yarn to make sure your gauge matches. If necessary adjust your needle size so you get the same number of stitches per inch as the original yarn gauge. Also, you would want your substitute yarn to have the same drape. For example if your original worsted weight yarn is more elastic, you want your substitute yarn to also be more elastic.