The rule says your strap, your belt and your shoes should agree. It is true, but narrower than people tell it: it holds for formal dress and stops holding almost everywhere else. Applying it to everything makes you dress worse, not better.
It comes from early twentieth century menswear, when the wristwatch was entering the serious wardrobe and had to behave like the other leather items. In that setting the rule has a precise logic: against a suit, the eye reads three points of leather — foot, waist, wrist — and if they disagree the reading breaks.
What agreeing actually means
It does not mean identical. I have watched people hunt for months for a strap in exactly the same brown as their shoes, and land on the worst possible outcome: two browns that are close but not equal read as a mistake, while two browns that are clearly different read as a decision.
Agreeing means same family, same finish. Dark brown shoe, brown strap — you do not need the same shade, you need neither of them pulling red while the other pulls yellow. Black shoe, black strap or a very dark chocolate.
Finish matters more than colour, and it is the part almost nobody looks at. A polished calf shoe with a matte pebbled strap clashes harder than black against brown does. They are two surfaces reflecting light differently, and the eye registers that before it registers the shade.
The metal, which shows more than the leather
If I had to keep one rule it would be this one, and it is not the famous one: your belt buckle and your watch case should be the same metal. A gold buckle with a steel case appear in the same frame every time someone sits down, and that gap is louder than two different browns.
It applies to the strap buckle too. On a gold watch, a steel buckle is the first thing I notice when someone offers me their wrist. It is a detail worth a few euros that ruins a pairing you built carefully.
Where the rule stops holding
With sports watches, immediately. A steel diver on a textile strap does not need to agree with anything: that watch was made for a setting where leather shoes do not exist. Looking for harmony there means applying a code to an object designed to sit outside it.
With casual dress, nearly always. If you are wearing sneakers, the rule has no reference point: a sneaker is not a leather accessory in the sense the rule means, even when it is made of leather. The same applies in summer, when shoes may be canvas and there is no belt at all.
And it stops holding when there is no belt. An obvious thing almost nobody accounts for: if your trousers have no loops, or you wear braces, there are two points of leather to coordinate instead of three, and the strap has far more freedom.
Then there is the practical argument, which outweighs all the others together. For most of the day nobody sees your wrist and your shoes in the same glance. They are a metre and a half apart. The rule matters when you sit at a table with your hands above it and one foot crossed below. At meetings and at dinner, in other words. Not walking down the street.
The minimum that actually helps
Anyone with a dress watch who does not want to think about this again needs two straps: one black, one dark brown, both in smooth calf, both with a buckle in the same metal as the case. Those two cover every combination of formal shoes in existence. A third strap is for pleasure, not necessity.
If the watch is your only one and you wear it at weekends too, the reasoning inverts: take the strap that works with the watch and stop worrying about shoes. A watch you wear constantly has to agree with itself, not with the day.
An opinion not everyone shares
The strap should never be the most formal element in the outfit. A polished alligator on a wrist coming out of a sports shirt is more out of place than a plain calf under a suit. Downwards is forgiven, upwards is not — that is the direction of the error, and it is the one I see made most often.
What I do not know
On dress codes outside Europe and the United States I do not have a view. I worked the American market for twenty-five years and I know that one, but conventions vary more than we imagine and I lack the direct experience to say what works elsewhere.
The short version
The rule holds for formal dress: same colour family, same finish, and above all the same metal between belt buckle and watch case. Outside formal dress it does not hold — and forcing it is the most common way to get a pairing wrong while believing you got it right.