Grading ruler and set of french curves.

No self respecting pattern maker likes to be without an accurate ruler. The best rulers for pattern making are made of strong clear plastic and have a grid across the whole ruler making it easy to accurately mark seam allowance. Clear grid rulers, such as the one pictured, are great for marking seam allowance on straight edges where you can get a completely parallel line. They are also ideal tools for grading patterns, hence they are often referred to as grading rulers.

Even around curves you can line up the grid of the ruler correct to the mm and make a small line marking, pivoting your ruler along the curve to create a dashed line. This provides an accurate guide for filling in the rest of the curve with a french curve ruler.

The images above show examples of grading rulers and french curves, though of course they come in many varieties and you are best to test out which type works best for you.

Below are some basic diagrams and instructions for using these tools to mark the seam allowance on your patterns:

1. Along straight lines of stitching you can use the grid ruler to mark parallel lines at your desired width of seam allowance. For the sake of the diagrams I am using a ruler with thick black lines at each cm marking (thin black lines every 2mm). So the seam allowance marked here is 1cm, but this will change depending on how you intend to finish your seams.
2. You can then use your ruler to make small dashed guidelines along the curve of the armhole and necklines.
3. The benefit of having a grid ruler is that you can use the full width of the ruler to mark in wider seam allowances and hem allowances.
4. You can use a french curve to connect the dashed guidelines together. You may find it easier to smooth this by hand instead.
5. One of the most important steps is to finalise seam allowance corners. The best way to make sure these corners are correct is to think about how it will be when you sew the seams. You will want the seams that you are sewing first to have nice right-angled corners, this makes matching up seams so much easier. It also makes it easier to check the patterns against each other.
6. You also want to consider how wider seam allowances will sit when sewn. Similar to darts, you are better to fold out the value of wider seam and hem allowances so that you ensure you are allowing enough fabric. If you picture the hem allowance folded up in the incorrect version, the seam allowance will angle inwards and will restrict the waist. In the correct version the hem allowance will fold up and sit nicely inside the body of the garment.

Grading ruler image originally from PerfectPatternAids. (Editor’s Note: This link had expired when it was checked on 7/10/12 and was removed)

Set of French curves, image from GraphicsDirect»

Pattern images from The Cutting Class.