Lug-to-Lug
Lug-to-lug is the vertical distance from the tip of the upper lug to the tip of the lower lug. It is this measurement, not the case diameter, that decides whether a watch will truly fit your wrist.
Most people look at the case diameter, yet comfort is governed by the lug-to-lug. If that distance overhangs the flat of your wrist, the lugs dig in no matter how thin the watch may be. A practical rule: the lug-to-lug should not exceed the width of your wrist. Slimmer wrists are usually happy up to roughly 44 to 48 mm lug-to-lug, which is why a 39 to 40 mm watch often wears better than a 42 mm one.