A watch case has two pairs of lugs at the 12 and 6 positions; the strap or bracelet attaches via spring bars (or fixed bars on military / vintage references) running between each pair. The lug width is the distance between the two parallel lugs in each pair, measured at the spring-bar level. The width determines which strap fits; a 20mm lug width takes a 20mm strap, period.
Standard sizes: 16mm, 18mm, 20mm, 22mm, 24mm are the common lug widths in modern watchmaking. The convention is even-number millimetres; odd-number widths (17, 19, 21mm) are rare exceptions. Rolex Datejust 36 / Day-Date 36 use 19mm as a notable exception (the 19mm width was original to the 1956 Day-Date and has been retained for parts compatibility). Vintage watches sometimes used 17mm; modern microbrand straps for these references are typically only available via specialist suppliers.
"The lug width is the only number on the watch that matters when you buy a strap. Everything else is opinion."- Watch retailer on strap selection
Modern men's watch convention: 20mm on 38-40mm cases (Submariner 41, Speedmaster Pro, modern dress watches at 39-40mm); 22mm on 41-44mm cases (Pelagos 42, larger pilot watches, modern dive watches); 24mm on 44mm+ cases (Panerai 47mm, AP Royal Oak Offshore 44mm+, oversized pilots). The lug-width-to-case-width ratio matters visually: a 20mm strap on a 40mm watch reads balanced; the same 20mm strap on a 44mm watch reads narrow.
Modern women's and dress watch convention: 16-18mm on 28-32mm dress cases (Cartier Tank Must, Patek Twenty~4); 18-20mm on 35-37mm dress cases (Patek Calatrava 5119, Vacheron Patrimony 36mm). Smaller lug widths visually communicate delicacy + dress positioning; oversized straps on small cases read as proportionally wrong.
For buyers, the practical guide: measure the watch's lug width with a digital caliper (most accurate) or a millimetre ruler (acceptable). Standard ruler from 0mm crisp to the inside edges of the lugs gives the spring-bar width. Most modern watch boxes / paperwork show the lug width; if missing, the brand website typically lists it. Aftermarket straps require matching mm only; fit is otherwise universal across brands. Quick-release spring bars (modern Tudor, Omega, IWC) make strap-changing tool-free; classic spring bars require a spring-bar tool (USD 5-15) to compress and remove.
