Glossary
Toile
A toile is a test garment sewn in cheap fabric to check a pattern's fit and proportion before cutting the real cloth.
Traditionally sewn in calico, which is where the American name — muslin — comes from. The fabric should behave like the intended cloth: testing a bias-cut silk dress in stiff calico tells you very little.
A toile answers fit questions. It cannot answer geometry questions — whether a seam matches, whether a notch lands in the right place, whether a size graded correctly — because those are answerable before anything is cut, and sewing a garment to discover them is the expensive way to find out.