How to Sew Curves Accurately: Necklines, Armholes, and Princess Seams

A straight seam is forgiving. A curved seam isn’t. When two curved edges have to fit together — a sleeve cap into an armhole, a princess seam shaping a bust, a neckline curving across the chest — the difference between a clean finish and a puckered one is almost entirely technique.

This guide covers the four scenarios where curves matter most for home sewers: necklines, armholes, princess seams, and the curved hem. The same underlying principles apply to all of them.

Concave vs convex curves

Two terms worth knowing:

• Concave = curving inward (like the inside of a bowl). A neckline is concave from the body’s perspective. Armholes are concave.
• Convex = curving outward (like the outside of a ball). The top of a sleeve cap is convex. A scallop is convex.

Most curved seams join one concave edge to one convex edge — for example, the convex sleeve cap fits into the concave armhole.

The trick is that concave edges have less circumference than the area they enclose, while convex edges have more. To make them meet smoothly, the convex edge needs to be slightly eased in, and the concave edge needs small clips to allow it to spread.

The fundamentals (apply everywhere)

Five things matter for every curved seam:

1. Stay-stitch fragile concave curves

A stay stitch is a single line of stitching just inside the seam allowance, run along the concave curve before assembly. It locks the curve at its sewn dimensions and prevents the bias-cut nature of curves from stretching out of shape during handling.

How: 12 mm from the cut edge, sew a single line of stitching along the curve, just inside the seam allowance. Stitch length 2.5 mm, at normal speed. No backstitching at ends.

This step is non-optional on necklines, armholes, and concave waistbands. Skipping it produces necklines that gradually grow over the course of the project.

2. Pin or clip carefully on the curve

Pin perpendicular to the seam line, with pin heads facing into the seam allowance so you can pull them out as you sew. On tight curves, use more pins, not fewer — every 2 cm is reasonable on a tight neckline.

Sewing clips (Wonder Clips, etc.) work as well as pins on most fabric, especially when one piece is much heavier than the other.

3. Use a shorter stitch length on curves

The default stitch length on most machines is 2.5 mm. On curves, drop to 2.0 mm. Smaller stitches give more control on tight curves — fewer “approximations” of the curve as straight segments.

4. Sew slowly

The temptation to keep your foot down on the pedal is strong. Resist it. On curves, slow stitching gives you time to adjust the fabric direction every few stitches. Use the speed slider if your machine has one.

5. Clip the seam allowance after sewing

Concave curves need their seam allowance clipped (snipped almost to the seam line) so the seam can spread when turned. Convex curves need their seam allowance notched (small wedges removed) so excess fabric doesn’t bunch when turned.

We’ll cover both below.

Sewing a neckline (concave)

A neckline is the most common curved seam in beginner sewing. It’s typically finished with one of three methods:

• Bias binding — a narrow strip of bias-cut fabric wrapped around the raw edge.
• Facing — a separate piece of fabric mirroring the neckline shape, sewn to the right side and turned inside.
• Self-bound — a wider hem folded inward to enclose the raw edge.

Whichever finish you choose, the technique is similar.

Step 1: Stay-stitch the neckline

As described above. Run a single line of stitching at 12 mm from the cut edge along the entire neckline.

Step 2: Apply the binding or facing

Pin and sew the binding or facing to the neckline along the seam line (typically 1.5 cm from the raw edge, or whatever the pattern specifies). Sew slowly, easing the fabric around the curve — don’t pull it.

Step 3: Clip the seam allowance

After sewing, clip the seam allowance at intervals of about 1.5–2 cm along the curve. Each clip is a small straight cut from the raw edge inward, going to within 2–3 mm of the stitching but not through it.

These clips allow the seam allowance to spread when the binding or facing is turned to the inside.

Step 4: Turn and press

Turn the binding or facing to the wrong side of the garment. Press carefully, rolling the seam slightly toward the inside so the seam line sits invisibly behind the front edge.

Step 5: Topstitch or hand-finish

Most necklines are topstitched on the right side, about 5 mm from the edge, to lock the binding or facing flat. Some couture finishes hand-stitch the inside edge of the facing for invisible finish.

Sewing armholes (concave joining a convex sleeve cap)

The classic challenge of garment sewing. The sleeve cap is convex — it curves outward, with extra fabric to “ease” into the slightly smaller armhole circumference.

Step 1: Run an ease stitch on the sleeve cap

Between the notches on the upper sleeve cap, run two parallel rows of long basting stitches (stitch length 4 mm) just inside and just outside the seam line. Don’t backstitch.

Step 2: Pull the bobbin threads

Gently pull the bobbin threads of both ease stitches to gather the sleeve cap slightly. The aim is to reduce the convex circumference until it matches the concave armhole, while keeping the gathering smooth — you should not see actual gathers, just slight ease.

Step 3: Pin the sleeve into the armhole

With the garment turned right-side out and the sleeve also right-side out, slide the sleeve into the armhole so right sides face each other. Match the notches: the front sleeve notch to the front armhole notch, the back sleeve notch (usually a double notch) to the back armhole notch, the shoulder seam to the top of the sleeve cap, and the underarm sleeve seam to the underarm bodice seam.

Pin generously between the matched points, distributing the eased fabric evenly.

Step 4: Sew with the sleeve side up

Stitch with the gathered sleeve cap on top — you can see the ease as you sew. Sew slowly, smoothing any potential pleats with a pin or fingertip just before they reach the needle.

Step 5: Press the seam toward the sleeve

Don’t press the seam open at the underarm — there isn’t space. Press the seam allowance toward the sleeve, then optionally clip a few small notches at the underarm to release tension.

For a full walk-through of garment fitting issues that affect armhole/sleeve construction, see our pattern fit adjustments guide.

Sewing princess seams

Princess seams shape a bodice with curved seams running from the shoulder or armhole down to the waist. The seam itself joins a concave curve (the centre piece) to a convex curve (the side piece) at the bust.

Step 1: Stay-stitch the concave centre piece

The concave curve on the centre piece (around the bust shaping) is bias-prone. Stay-stitch along the seam allowance edge as described above.

Step 2: Mark match points

Princess seam patterns typically include match dots at the bust point and notches at the waist. Transfer these accurately. The match between the convex side piece and concave centre piece depends on these points lining up.

Step 3: Pin generously at the bust point

Pin the seam at the bust point first — the most curved area. Then pin upward toward the shoulder/armhole, easing the convex side piece into the concave centre piece. Pin downward to the waist, doing the same.

Step 4: Sew slowly on the curve

Stitch with the convex piece on the bottom (against the feed dogs). The feed dogs help ease the convex curve while you guide the concave one on top. Sew slowly through the bust curve.

Step 5: Clip the seam allowance

Clip the concave seam allowance at intervals of about 1.5 cm around the bust curve. The clips let the seam allowance spread when pressed open or to one side.

Step 6: Press

Press the seam open if the fabric allows; otherwise press to one side (typically toward the centre).

A well-sewn princess seam has no puckers, no pleats, and a smooth curve from shoulder to waist. It takes practice — most beginner sewers do their first princess seam imperfectly and the second much better.

Sewing a curved hem

A curved hem (on a circle skirt, A-line skirt, or curved shirt tail) presents a different challenge: when you fold the hem to the inside, the outer edge has more circumference than the inner edge it folds against. The excess wants to pleat.

Two approaches:

Approach 1: Narrow hem with ease stitching

Sew a line of long basting stitches 5 mm inside the hem allowance edge. Pull the bobbin thread gently to ease the outer edge slightly. Press the hem allowance up and finish with a narrow double-fold (5 mm + 5 mm).

Approach 2: Bias-bound hem

Apply bias binding to the raw hem edge, then fold up and stitch. The bias binding handles the curve without needing to ease the fabric.

For tight curves (like the bottom of a circle skirt), bias binding is significantly easier than trying to ease a deep folded hem.

Common curved seam mistakes

Puckers along the curve

Most likely: not stay-stitching first, or sewing too fast. Re-sew the curve with stay-stitching and slow speed.

The garment piece is wonky

Check that the curve was clipped after sewing. Without clips, concave curves can’t lie flat when turned.

The seam shows through to the right side

The seam allowance wasn’t pressed, or wasn’t graded (trimming the two layers of seam allowance to slightly different widths so the bulk doesn’t ridge through).

Pleats forming as you stitch

You’re trying to flatten the fabric as you sew. Stop fighting it — pin generously, sew slowly, and let the fabric move as the needle works through it.

Practising on scrap

Cut two pieces of cotton fabric:

• A 15 cm square with a 10 cm semicircle cut into one edge (concave).
• A 15 cm square with a 10 cm semicircle bumping outward from one edge (convex).

Pin them together so the curves face each other, sew them, clip the seam allowance, and press. Repeat once.

Twenty minutes of curve practice teaches you more than a full project where curves are one of many things going on.

Closing thought

Curves are where the difference between beginner and intermediate sewing shows most clearly. The technique is small — stay-stitch, pin, sew slowly, clip, press — but the discipline of doing it on every curved seam is what produces a clean finish. Skip any one step and the curve shows it.

Once you’ve sewn three or four projects with deliberate curve handling, the techniques become automatic. After that, every curved seam in every later project benefits.

For specific projects that practise these techniques, our 25 beginner sewing projects path introduces curves gradually — necklines around project 11, full princess seams around projects 16–20.