Muslin / Toile

Muslin (the American term) and toile (the British and French term) refer to the same thing: a rough, disposable test garment, usually sewn from cheap, plain fabric — often the fabric literally called muslin, a lightweight, inexpensive plain-weave cotton — to check that a pattern actually fits and works before committing real time and real fabric to it.

It's not meant to be a finished garment. Seams often go unfinished, some details get skipped entirely if they don't affect fit, and it's expected to get marked up with pen and pins as you check and adjust it on the body or a form. What it's checking is everything that's hard to judge from a flat pattern: whether darts point where they should, whether there's a pull line or drag line anywhere signaling too little ease, whether a neckline actually sits where it looks like it should on paper.

I skip a muslin only for patterns I've already made and fitted before, or for genuinely simple, non-fitted shapes with almost no risk of a fit surprise. For anything new, fitted, or made from fabric I don't want to risk, it's the step that saves the actual garment from an unpickable mistake.