The lug hole and the pin

Everyone measures the wrong thing. Ask a collector if a watch will take a new strap and he reaches for the lug width, the eighteen, twenty, twenty-two millimetre number stamped into every spec sheet. Fair enough, it matters. But it is not the thing that decides whether a watch can carry a second object on the same wrist. The thing that decides that is a hole most people have never once looked at. It is about a millimetre across, drilled into the side of the lug, and it quietly governs the entire question. Every watch carries two specifications you can see and one you almost never notice. The visible one is the lug width, how wide the opening is. The invisible one is the interface, whether there is a door at all.

What a lug hole actually is

Let me make the case for the most ignored detail on your watch. The lugs are the four arms that reach down from the case to hold the strap. Inside each pair sits a bar, and that bar has to seat into something. That something is the lug hole, a small cavity drilled into the inner face of the lug. On a lot of tool watches the hole goes all the way through, so you can see it from the outside and push a pin out with a toothpick. Watchmakers call those drilled lugs. On many dress watches and most modern sports models the hole is blind, hidden on the inside only, for a cleaner case flank.

For a hundred years the bar that lived in those holes was the spring bar, a sprung telescopic pin you compress to fit. Smartlet does something different, and this is where the lug hole stops being trivia. Instead of a spring bar, a solid pin seats directly into the lug holes, a 1.2 millimetre tip on each side, sized across an 18 to 24 millimetre fit kit. The watch is held by its own lug holes, not by a borrowed bar.

The gate, and who gets locked out

Here is the honest part, the part a brochure would skip. Not every watch passes through that gate, and the reason is almost always that the maker replaced the standard lug-hole interface with something proprietary.

Panerai uses screw-in bars, bolted rather than sprung, a closed system. The Tudor Pelagos FXD has a fixed bar machined into the case, no holes to work with at all, by design, because military divers did not want a strap that could pop off. Garmin's QuickFit and Hublot's one-click systems swap the whole interface for a branded mount. In each case the lug hole as a universal connection point has been engineered away, and a pin has nowhere to live.

So the answer for those is a clean no. I would rather tell you that up front than sell you a fantasy. It is also why a rugged option sometimes means a Garmin on its own strap rather than under the watch.

Then there are the watches with no lugs to speak of, which sounds like the end of the road and is not. An Apple Watch has no lugs at all, so it rides on a dedicated adapter rather than a pin, which is the basis for wearing a Rolex and an Apple Watch together. A Tissot PRX hides its strap inside an integrated case, so it too gets its own adapter. A resin G-Shock works the same way, through an adapter rather than the case. The lug hole rule has exceptions, and the exceptions are deliberate engineering, not luck. The full Apple Watch compatibility sits on its own page.

Width still has the final word

Once the interface is sorted, the old number returns to settle the rest. The pin kit spans 18 to 24 millimetres, so an 18, 19, 20, 21, 22 or 24 millimetre watch is in range. Twenty-three is the odd gap, not yet supported, with that size arriving in July 2026. Very small lugs, the 12 to 17 millimetre world of slim dress pieces, fall outside the standard range and generally do not qualify. If you want the full treatment on the width question, that is its own subject, and it pairs with this one rather than repeating it, the way the lug width guide handles the distance while this handles the hole.

This is the same logic whether you are adding your first smartwatch to a serious watch or composing the setup on purpose.

The lug width is printed on the spec sheet. The lug hole is where compatibility actually lives. Go and look at yours, then the brand compatibility index and the collection tell you the rest.