I love yarn....all yarn. There is a purpose for every type of yarn created. Some purposes are more obvious than others and some certainly more practical. You wouldn't use a gorgeous cashmere or silk to knit dishcloths nor would you waste your precious knitting time using a 2 cent acrylic to knit eveningwear. Selecting the right yarn for the purpose is vital to a great outcome. While most patterns promote a specific yarn, it's not always possible or economical to use it. If you know what to look for, it will be easy to substitute others yarns. You will need to pay close attention to the yarn weight and gauge, texture, and quantity. The first quality, yarn weight, is crucial so that the resulting garment will be knit in the correct gauge and therefore correct garment size. This can be tricky but the easiest way to tell the weight compatibility is to look at the yarn label. It will tell you how many stitches and rows it takes on a certain size needle to equal four inches. If you can match the gauge information on another yarn label you have a winner. The second crucial quality for a good substitution is the yarn's texture. If you want the garment you are knitting to closely resemble the one in the pattern, you will need to use a yarn with a similar texture. Knitting in a nubbly or fluffy yarn, when a smooth one is recommended, will totally alter the outcome of the garment. Be consistent. If multiple colors are called for, be certain they are of the same weight so the gauge and resulting texture will be consistent. The third aspect to consider is the yardage. There can be a wide variety of yardage in skeins with the same weight. Provided the gauge matches the substituted yarn, purchase enough yardage in the same dye lot to complete your project. With the freedom to substitute, your chance to create a unique garment is in your power.