The most useful skill an owner can learn
A strap or bracelet change is the single highest-impact thing you can do to a watch you already own. The same watch reads dressy on dark leather, vintage on suede, sporty on rubber, and casual on fabric — one head, several personalities, for the price of a strap. Learning to change straps yourself is worth the small effort, both for the versatility and because the alternative, a jeweller's bench, is inconvenient for something you may want to do weekly. It is also where the most common avoidable scratch happens, so it is worth doing with the right tool and a little care.
Lug width: the one number that matters
Straps attach between the lugs, and the gap between them — the lug width, measured in millimeters — is the only compatibility number you need. It is almost always even (18, 19, 20, 21, 22mm), and a strap of that width fits any watch with that gap, across every brand. Measure the gap with calipers or a ruler if you do not know it; ordering the wrong width is the most common first-time mistake. The companion figure is the taper — a 20/16 strap is 20mm at the lugs and narrows to 16mm at the buckle — which affects proportion and buckle compatibility but not fit between the lugs.
Changing a strap without scratching the watch
Straps are held by spring bars: sprung pins with a shoulder at each end that seats into a hole in the lug. The tool for the job is a spring-bar tool, a few dollars, with a forked end that catches the bar's shoulder. The procedure: slip the fork between the strap and the lug, catch the spring bar's shoulder, compress it inward until that end clears its hole, and angle the strap out. Reverse to fit the new one — seat one end first, then compress the other and let it spring into its hole, and tug the strap to confirm both ends are locked.
The scratch happens when the tool slips off the bar and skates across the lug. Two defenses: work slowly with the fork firmly seated on the shoulder, and on watches with drilled lugs — a small hole through the lug, visible from outside — push the bar in from the outside with the tool's pin tip instead, which keeps metal away from the case entirely. Drilled lugs make strap changes trivial and safe, which is why enthusiasts prize them.
Spring bars are the cheap component your entire watch hangs from. They wear, weaken, and occasionally fail — and a failed bar drops the watch. Replace them periodically, use quality double-shouldered bars on anything valuable, and on a heavy watch consider the small upgrade of fatter bars. Pass-through fabric straps (NATO-style) hedge the risk: the strap runs under the caseback, so even if one bar fails the watch cannot fall.
How a watch should fit
Fit is governed less by case diameter than by the lug-to-lug measurement — the distance tip to tip across the lugs — which must sit within the flat top of your wrist without the lugs overhanging the edges. A watch whose lugs hang off your wrist looks and wears too large regardless of its diameter, which is why two 40mm watches can fit completely differently. On the wrist, a watch should sit so it does not slide more than a little — snug enough to stay centered on the top of the wrist, loose enough to admit a finger and to move as your wrist swells through the day (wrists are measurably larger in heat and late in the day). For bracelets, the best clasps offer micro-adjustment to tune that final couple of millimeters; for straps, the right hole should leave a little tail through the keeper. A correctly sized watch is one you stop noticing — present but not pinching, secure but not tight.
The strap is half the watch and the easiest half to change, so learn to do it: know your lug width, use a proper spring-bar tool, respect the drilled lug, and never trust an old spring bar on a watch you would hate to drop. Then size it by the lugs and the wrist, not the spec sheet — because the best-fitting watch is the one that disappears on your arm, which no diameter on paper can predict.