Muslin (an unbleached, plain-weave cotton fabric) is so commonly used for mock-ups of patterns that the mock-up itself is referred to as "a muslin". (A mock-up, by the way, is a very preliminary sample you'll make to see if the pattern or styling is working. Unlike a prototype, which is constructed in a manner very close to the final product, a mock-up does not have facings, zippers or closures, or hems)
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You can use any fabric for your mock-ups and samples. This is the traditional cloth, used because it's readily available and inexpensive.
If the garment you are working on will be made of a knit fabric, your samples should be made with a similar knit.
If the garment you're working on will be made with a woven fabric, use a woven (like muslin) for the mock-up; but any plain-weave woven will do for the initial sample. Even an old sheet will do!
But it should be light colored and even weave, so that the grain lines aren't confusing and you can easily see whatever markings you need to make on your sample.
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