Muslin (toile)
A trial garment sewn in inexpensive fabric to check the fit and proportions of a pattern before cutting the final fabric.
It is called a muslin in the US and a toile in the UK. It shows problems of fit and balance that are hard to see on paper, and the corrections are transferred to the pattern. It is common in made-to-measure and couture work and for new base patterns; in production the same role is played by the sample.
Example
Before cutting an expensive wool, a patternmaker sews the jacket in plain cotton muslin, adjusts the shoulders and sleeves on the body and corrects the pattern accordingly.