The one tool that makes dart marking faster and far more accurate on the cutting table.
Marking darts by hand with a straight ruler means measuring twice for every dart: once for the placement and once for the intake. On a busy cutting table that is where the errors creep in.
A dart scale carries the common intakes as fixed marks, so you place it once and trace. It is the smallest tool in the kit and the one that most reliably saves a recut.
Use it with the pattern flat and the fold line square to the table edge. If the fold is off, every dart on the piece inherits the error.