An essential element of a pattern makers tool box is to have a really good pattern notcher. Pattern notchers are used to make small notches in the edge of cardboard and paper patterns to mark seam allowance and positions of darts and pleats.
The picture below illustrates where and how a notcher is used on paper and cardboard patterns to mark the seam allowance. For more information on how notches are used when sewing seams you can visit the post, Fundamentals of Sewing: Seams»
Pattern notcher image from MacCulloch&Wallis»
Technical drawings from The Cutting Class»