The lug attachment is the mechanical system that connects the strap or bracelet to the watch case. There is no single universal system: each manufacturer has developed its own solutions, and the choice affects durability, reversibility, and ease of replacement.
The most widespread system remains the one with spring bars — the elastic pins that insert into the lugs. Traditional spring bars require a dedicated fork screwdriver to be removed; quick-release spring bars feature a small lateral lever that allows hand removal without tools. From the late Nineties onward, virtually every manufacture has introduced a proprietary variant of this concept: Hublot calls theirs One Click, Maurice Lacroix Easy Strap Exchange, Jaeger-LeCoultre Presto Spring. The underlying mechanism is always the same; what changes is the interface and, sometimes, compatibility with third-party straps.
The lugs themselves vary in geometry and construction. Fixed lugs are cast or machined as a single block with the case, as on the Tudor Pelagos FXD: maximum robustness, no articulation that loosens over time. Teardrop lugs, inherited from historical references like certain 1940s Audemars Piguet models, have a narrower section at the base that facilitated mounting hand-cut leather pads. T-bars connect the strap directly to the case back rather than the sides, a solution adopted on round cases where side lugs would compromise the lines.
On pocket watches, the attachment point is called the bow: the ring, soldered or threaded onto the case pendant, where the chain attaches. A loose bow indicates metal wear or seat degradation; it can be partially restored, but on fine pieces this is a detail a collector checks before purchase.
From the bench, I've seen breaks almost always on spring bars undersized relative to the case weight, and loosening on first-generation quick-release systems, where the lever lost tension after a few years of intensive use. Screw-based systems — like the screwed strap links found on some sports cases — remain the most stable over time, provided the screws are stainless steel and not soft metal.