Aaron Dennison was one of the "Fathers" of the American watch industry, and he invented several standards of measuring watches and mainsprings. The Dennison Gauge for watch sizes is based on a size A being 1" and for each additional size larger, you would add 1/16". The most common sizes were N (1 11/16), which is close to 18s and L (1 10/16), which is close to 16s. Used primarily by the E. Howard Watch Company (of which Dennison was a founder). Dennison Gauge is still used for the measurement of mainsprings for American pocket watches.