Glossary
Grade rule
A grade rule is the instruction that says how far a given point on a pattern moves when the pattern changes size.
In most systems a grade rule is a pair of offsets — so much across, so much up — applied to a numbered point, and a full garment is a table of hundreds of them.
That works while sizes progress evenly and starts failing at the extremes, where a body does not grow the same way in every direction. Widths, lengths, shoulders and necks all grow at different rates and none of them linearly, which is the difference between grading a pattern and scaling one.
The alternative is to hold the construction rather than the offsets and re-solve the whole pattern against a new size chart, so the shape holds at 3XL the way it holds at M and a non-linear progression is not a special case.