French Curves, Armhole Rulers and D Curves Explained

Eight Needles Journal · Patternmaking

A shaped ruler offers a family of curves, not a complete pattern solution. An armhole ruler or D curve helps connect points smoothly, but the points themselves must come from your measurements and drafting method.

Bunka D curved ruler — The D curve provides tighter and gentler sections along the same edge. Match a section to your plotted line.

Compare the curve, not just the name

Tool	Typical task	What to compare
Bunka D curve	Deeper neckline, armhole and crotch curves	The short section that follows your plotted points
Bunka H curve	Gentler skirt and trouser side seams	Whether the long, shallow bend suits your line
Armhole ruler	Connecting points around armholes and sleeve curves	Its actual outline and the drafting method you use
French curve	A selection of curved edges for drawing	Shape, size and any useful graduations; sets differ

For Bunka, D refers to “Deep” and H to “Hip”. The names suggest a purpose, but they do not specify the finished garment size. An armhole ruler does not replace body measurements, a block or the instructions for drafting that armhole.

Fit a section of the edge

Rotate and slide the ruler until part of its curve passes naturally through your reference points. You do not need to use the whole edge. Trace lightly and continue with overlapping sections so the joins can be checked.

Try a short section before buying more curves

Choose one awkward area from a pattern you use often. See whether your existing curve can follow its beginning, middle and end by repositioning. If it can, a second similar ruler may add little. If you repeatedly struggle with a long, shallow hip line, an H curve offers a different shape rather than another version of the same deep curve.

A curve set can be useful if you want both shallow and deep outlines together. For example, Clover 25-051 combines those two shapes with a 15cm mini ruler and a case. That is a different format from a single D curve; compare the useful edges rather than counting the pieces.

Look at the direction of the line

A smooth-looking curve can still meet the next line at an awkward angle. Check the transitions at shoulder, side seam or other joining points specified by the pattern. Compare adjoining pieces before accepting the final shape.

Drawing a curve is not checking its length

After redrawing an armhole or neckline, measure the new seamline and check its matching piece. A smooth outline can still be too long or too short. Use a flexible tape, suitable graduated ruler or measuring wheel to check the required sections; do not assume every unmarked template edge measures length.

Keep the pencil point close to the edge and use light pressure. Thick, repeatedly drawn lines make it harder to decide which line to sew. These drawing rulers are not cutting guides: a blade can nick the edge you need for smooth drafting.

Preserve the original

Work on a traced copy when experimenting. Label the new curve and keep the original line visible until you have checked the change. That record lets you reverse an adjustment instead of trying to remember which version you started with.

A practical example

Three plotted points define a curved edge. Find the section of the ruler that joins them naturally, rather than forcing all three against its longest curve. Trace lightly and inspect the transition into the neighbouring straight line before accepting the final shape.