Lug-to-lug distance is the measurement that truly matters when a watch needs to sit on your wrist: not the case diameter, but the total length from the apex of the upper lug to the apex of the lower lug. Two 42 mm watches can have lug-to-lug measurements of 47 mm and 51 mm respectively, and they behave radically differently on the wrist. The one with the shorter lug-to-lug curves better over the carpus, stays centered, doesn't encroach on the tendon. The other protrudes, moves around, feels different in weight even at the same gram count.
At the bench I learned to measure this distance before even the diameter, especially with vintage pieces from the Sixties and Seventies, where the lugs were often elongated to follow the fashion of covered wrists. A German Beobachtungsuhr with 55 mm diameter can have lugs so short that it becomes surprisingly wearable; a Swiss dress watch at 36 mm with bayonet lugs can instead exceed 44 mm lug-to-lug and look enormous on a thin wrist.
The measurement is performed with a sliding caliper: you rest the points on the extremities of the lugs, parallel to the vertical axis of the case, without including the strap or bracelet. Some manufacturers declare this measurement in the technical specifications, others don't. When it's missing, you must measure it directly. In the Milano Straps database every watch entry reports the lug-to-lug verified on the physical piece, not the one declared.
The shape of the lugs interacts with the lug-to-lug distance: straight lugs project the entire measurement beyond the case, curved lugs partially absorb it by adhering to the wrist. For this reason lug-to-lug alone is not enough: it must be read together with the lateral profile of the lug. A lug-to-lug of 48 mm with strongly curved lugs can be more comfortable than one of 45 mm with perfectly flat lugs. It's a geometry, not an isolated number.