Watch lugs are the projections that extend from the case and hold the strap or bracelet. Most watches have two lugs at the top and two at the bottom. A spring bar, screw bar, or fixed bar spans the gap between each pair.
Lug width tells you strap size
Lug width is the inside distance between the two lugs on one end of the case. It is usually measured in millimeters. A watch with a 20 mm lug width normally takes a 20 mm strap at the case.
Measure the gap itself, not the outside edges of the lugs. Even a one-millimeter mismatch can create a poor fit. A strap that is too wide may bunch or rub. One that is too narrow can slide sideways and expose the spring bar.
Lug-to-lug affects wrist fit
Lug-to-lug length is different. It measures from the tip of the upper lugs to the tip of the lower lugs. This dimension helps explain how much space a watch occupies along the wrist. Two watches with the same case diameter can wear very differently if one has longer or straighter lugs.
Lug shape matters too. Curved lugs can help a case follow the wrist. Straight or extended lugs may make a watch feel larger. Integrated bracelets and unusual lug shapes can limit replacement options.
Be careful when changing a strap
Spring-bar tools work close to the case, so slipping can scratch the lugs. Longines instructs owners to locate the spring bars where the strap connects to the lugs, compress the bar carefully, and protect the watch on a cloth. If access is tight or the watch is valuable, a watchmaker is the safer choice.
- Confirm the exact lug width.
- Use the correct spring-bar diameter and length.
- Check that both ends of each bar are fully seated.
- Give the strap a gentle pull before wearing the watch.
For more strap basics, see how to measure watch strap size.
Sources: Longines strap-changing guide and Seiko specifications showing lug width.