Definition
A pattern block, called a sloper in the US, is a basic close-fitting pattern with no style details, from which garment patterns are developed by flat-pattern methods.
A block describes the body plus wearing ease, and nothing more: a bodice with darts, a straight skirt, a fitted sleeve, a trouser. It usually has no seam allowances. Designers trace it and change it to create styles: moving or combining darts, adding flare by slashing and spreading, lowering a neckline, adding design ease, and finally adding seam allowances.
Common blocks:
- Bodice block, front and back, with bust and waist darts.
- Skirt block, straight from hip to hem, with waist darts.
- Sleeve block, a one-piece set-in sleeve matched to the bodice armhole.
- Trouser block, with front and back crotch curves.
- Knit blocks, with little or no ease and no darts.
Pattern companies draft one block per size. A custom block drafted from your measurements, and refined with a toile, is the basis of made-to-measure sewing, since every style built on it inherits the fit.
That is how Sewly works: its drafting engine draws every piece of a pattern from flat-pattern blocks with your measurements and the right ease for the garment, so the same inputs always give the same pattern. More in how Sewly drafts a pattern.