A watch case is a small metal circle. The strap is the long stripe that runs across your wrist, and it is what meets your sleeve, your cuff and your skin. That is why changing only the strap can make the same watch look like a different watch: a casual one, a dressy one, a quiet one, a shiny one.
This piece compares the three straps you will meet most often on classic analogue watches: leather, a steel bracelet (a strap made of metal links) and steel mesh (a fine woven metal band, often called Milanese). It gives a simple way to match each one to what you are wearing, and what to check before you buy.
Leather is soft and matte. It reads as warm and a little classic, and it takes on the character of the wear over time. Black leather leans dressy: it matches black shoes and a dark belt. Tan or brown leather leans casual and looks best with denim, knitwear and suede. The trade-off is water and sweat: leather does not like either, and a strap worn every day will need replacing sooner than a metal one.
A steel bracelet is the all-rounder. The links catch light in small, even lines, which gives a sporty, tidy look that sits happily with a tee and jeans and still works under a shirt cuff. It is heavier on the wrist, and it is the easiest to resize, because a link can be taken out.
Steel mesh is the soft metal option. Because it is woven rather than linked, it drapes smoothly and looks almost like a fabric. It reads as sleek and slightly dressy, and it pairs well with plain, clean outfits. Check the clasp before you buy: some mesh straps adjust to any length, others need a tool.
Leather warms an outfit up; metal tidies it up.
Choose one of four straps and one of four outfits. The drawing shows the strap on a plain round watch, and the scale on the right shows how casual or dressy the strap is (black marker) against what the outfit asks for (red marker). The closer the two markers, the easier the match. This is a rule of thumb, not a law, so the notes also say when to break it.
A reliable shortcut is to match the strap to the thing nearest it. With leather, repeat the colour of your shoes or belt: black with black, tan or brown with brown. With metal, repeat the metal of anything else you are wearing, such as a belt buckle, a ring or a chain, or keep to one tone and let it be the only shiny detail.
The other quick check is the cuff. A strap that is very wide or very thick can stop a shirt sleeve from closing smoothly, and a slim leather strap slides under almost any cuff. A steel bracelet adds a little height at the wrist, so it is worth trying with the sleeve you will wear it under.
Rules are made to be bent. A tan leather strap with a dark suit can look relaxed in a good way, and a steel bracelet with a black-tie look can look sharp if the case is slim and the rest is plain. The scale above shows the easy path; the stretch path works when you decide it on purpose.
If you already own a watch you like, a new strap is the cheapest way to change it. Look at the width of the gap between the lugs (the two arms that hold the strap), measured in millimetres: common sizes are 18, 20 and 22 mm, and the strap must match. Many straps now use quick-release pins, so the swap takes a few seconds with no tools. Second-hand watches and straps are a fair way to try a look before committing to it, and re-wearing what you own always counts.