I've been making lots of muslins lately as part of my patternmaking certificate. These test garments are usually created from muslin fabric, hence the name, and they are created to check fit. Muslin is an unbleached cotton fabric, and I get mine at the fashion design bookstore at my school. It comes in several grades, ranging from a fine to coarse weave.
The idea is that you choose a woven fabric that is similar in weight and drape to the fabric that you will ultimately use on your finished garment.
I was pulling a long night this past weekend, working on my final projects as the class draws to a close, and I ran out of muslin. I had enough to cut out the bodices in one grade, the arms in a rougher grade, which matched the back of the skirt. The front skirt was cut out from a white cotton-poly I had inherited from my mother-in-law's stash.
The finished result was quite a hodge-podge, as you can see from the picture, and the professor had a good laugh. But this was good instruction for me on why it's best to make your test garment from all the same fabric. Muslin creases easily and will hold darts and seams with just a few pins. The poly on the other hand had a different drape and refused to be molded, fighting me with every pin and needing lots and lots of pins.
Not everyone uses cotton for their muslins. There may cases where you would want to use an inexpensive fabric more similar to your fashion fabric. For instance, I designed a silk chiffon dress and used polyester chiffon to test it. You could do the same with charmeuse, satin, and so forth. And of course, for knits, you will want to use a knit to test your pattern. I just recommend sticking with lighter colors that can be marked easily as you make adjustments.
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Have you ever worked with muslin, or a different fabric to try out a pattern? I would love to know!