A block — called a sloper in North America — is a plain pattern with no design features, no seam allowance and only the darts needed to fit a specific body.
What it is for. It is a starting point rather than a garment. Once you have a bodice, sleeve, skirt and trouser block that fit, you draft designs from them by moving darts, adding fullness, changing necklines and reshaping seams. Everything is drafted from a shape you already know fits, so the fitting work is done once rather than on every project.
The second use is diagnostic. Lay your block over a commercial pattern, matching the centre front and the bust level, and the differences show you where the design ease sits and which adjustments that pattern will need. It turns a fitting session into a measurement.
How they are made. Either drafted from your own measurements using a drafting system, or developed by fitting a very simple pattern in calico over several rounds and transferring the corrections to paper. Draping on a stand is a third route.
Expect several toiles. A block is only worth having if it genuinely fits, and that takes iterations. Work in a plain woven calico of similar weight to what you will use, mark the changes on the fabric, and transfer them each time.
Store it on card or heavy paper, labelled and dated, and check it against your measurements every year or so. A block fits the body it was made for at the time it was made.
Questions people actually ask
Do I need a block to sew?
No. It is a tool for drafting and for diagnosing fit, and most people sew for years without one.
Does a block include ease?
Only minimal wearing ease. Design ease is added when you draft a garment from it, which is what keeps the block useful across styles.
Last reviewed 2026-08-11