Have you ever wondered how multi-sized patterns are developed? For the home sewist, there are simple ways to grade a pattern up or down a size, but pattern designers need to have an understanding of pattern grading rules to correctly grade designs for different sizes.
Grading does not take into account the difference in body shapes. You can't take a pattern designed for a petite woman and grade it up to fit a plus-size woman; the resulting garment would need extensive alterations.
Likewise, know that when you add or subtract for girth, the length of the garment will also need to change in proportion, regardless of the height of the individual.
So if you want to design a pattern for women in regular sizes as well as petite and plus, you'll need three different grade rule libraries for the same design.
Only halves or quarters of the garment are dealt with at one time, so differences are divided by two or four as appropriate.
An increase of an inch (or any amount) needs to be added in proportionally. This is why we split the pattern piece into four pieces in the cut-and-spread method.
An adult body can roughly be segmented into eighths, and there are a few points on the body where you can apply length grade rules proportional to height.
Coincidentally, these grade rules can also be expressed in eighths.
This means that if you were grading a blouse pattern that hit at the waist, the length would need to increase by ¼" for every 1" in added height. (As noted previously, this doesn't take into account any increases in girth.)
On a blouse, girth would be added to the chest, waist, neck base, armscye (or armhole) circumference, as well as the upper arm. The grade specification for each will maintain the proportions of the design as it increases or decreases in size.
Have you determined pattern grading rules for any of your designs? Let me know in the comments!