Add and verify seam allowance

What you are trying to accomplish

Tell PatternForge whether your traced outline is the sewing line or cutting line, then show the other line at the intended distance. For a sewing-line outline, PatternForge adds a cutting line outside. For a cutting-line outline, it can show the sewing line inside.

Why it matters

The sewing line controls the finished shape. Seam allowance is the material between that line and the cut edge; it is consumed during sewing and does not add wearing ease.

Choosing the wrong line meaning can create a double allowance or remove one that was already included. The amount and corner treatment also depend on construction, fabric, curves, hems, facings, and personal or production practice. PatternForge does not choose a universal correct width.

The current main control applies one allowance magnitude around the selected piece. It does not provide a complete per-edge construction specification, so special hems and differently finished edges still need deliberate handling.

Sewing line

Cutting line

Step by step

1. Select the piece and open Shape or Get it ready.
2. Answer Which line did you trace?
   • Choose Cutting line if you followed the outer line that will be cut and the allowance is already inside it.
   • Choose Sewing line if you followed the stitch line and still need a cutting line outside it.
3. If you are unsure, use I'm not sure — leave this unfinished and check the source wording. Do not mark the choice complete until the source confirms it.
4. Open Seam allowance.
5. Read the direction sentence before changing the value. Add outside belongs to a sewing-line outline. Show inside belongs to a cutting-line outline.
6. Use the minus and plus controls until the displayed magnitude matches the approved construction decision. Do not copy an allowance from another edge or pattern without checking how this seam will be sewn.
7. Choose a corner treatment—Miter, Round, or Bevel—based on the intended geometry and inspect every sharp or concave area.
8. For a true cut-on-fold piece, confirm the fold edge receives no seam allowance.
9. Zoom around the full perimeter. Look for spikes, gaps, loops, pinched corners, and places where the inner or outer line crosses itself.
10. Preview the export with the intended line enabled. Read the automatic piece-information text to confirm which line should be cut and which should be sewn.

What success looks like

• The outline is explicitly identified as a sewing line or cutting line.
• The displayed second line is on the correct side of the outline.
• The chosen magnitude comes from a known construction decision.
• Corners and curves produce a continuous, usable result.
• A real fold edge carries no allowance.
• The export text and visible lines agree about where to cut and sew.

Common problems and recovery

The allowance appears on the wrong side

Recheck Which line did you trace? If the outline is the sewing line, use Add outside. If it is the cutting line, use Show inside. Do not fix a wrong line meaning by merely changing the number’s direction.

The allowance has spikes or gaps

Repair the base outline first. Use Shape check to find crossings, duplicate points, tiny segments, or hooks. Then reapply the allowance and compare corner treatments.

A sharp corner looks clipped or exaggerated

Try the appropriate Miter, Round, or Bevel treatment, but judge it against how the seam is constructed and turned. Some shaped corners need manual truing beyond a uniform offset.

A curved hem looks unusable

A wide, uniform turn-up may not be suitable for that curve. PatternForge’s piece-wide allowance does not design a facing, eased hem, or shaped hem extension. Resolve the construction method separately.

The piece already included seam allowance

Set the outline to Cutting line. Use the inner line only as the sewing reference. Adding another outside line would double the allowance.

Removing allowance also removes a useful sewing reference

First confirm which line is the stored outline. The base outline remains; the generated companion line is what the allowance control adds or removes.

Different edges need different allowances

The primary UI currently applies one piece-wide value. Do not claim that a uniform setting represents edge-specific production instructions. Record exceptions clearly and resolve them before marketing or manufacturing use.

Quick safety check

Display both lines and point to them:

• Which one is the sewing line?
• Which one is the cutting line?
• Was the allowance already present in the source?
• Does every corner remain sewable?
• Is the fold edge free of allowance?
• Does the export’s written instruction match what you see?

Related tools and next step

Review cut-on-fold behavior, then use Define and match seams to compare sewing relationships. Run Shape check before Pattern Check whenever an offset looks irregular.