Brellum Duobox 39 Triple Calendar Moonphase Chronometer Arrives in Blue

Brellum keeps doing its thing. The indie brand from the Swiss Jura, founded in 2016 by fourth-generation watchmaker Sébastien Muller, has quietly built one of the more charming small-batch lineups in the independent space. The Duobox series sits at the heart of it, and the latest drop is the Duobox 39 Triple Calendar Moonphase Chronometer in blue. It's not a whole new watch, but it's a compelling new reason to look at one of the best-proportioned complicated watches at this price level.

The original Duobox launched as a retro chronograph in a chunky 41.8mm steel case with piston-style pushers and a COSC-certified Valjoux 7750 inside. Muller then pulled that idea in a tighter, more wearable direction, landing at 39.5mm with the current generation. The Triple Calendar Moonphase version adds day, date, month, and a moonphase display on top of the chronograph functions. That's a lot to pack into 39.5mm, and Brellum pulls it off without the dial feeling crowded.

The blue colorway is the news here. It joins what has been a mostly warm-toned roster for the Duobox TC lineup, and it works. The vintage-inspired aesthetic of the Duobox translates well to blue, giving it a slightly dressier personality while keeping all the sporty chronograph bones intact.

What's Actually on the Dial

Brellum leans hard into the retro playbook with the Duobox layout. You get subsidiary dials arranged in a classic tri-compax style, the kind of arrangement that nods directly to mid-century Swiss chronographs. The triple calendar information, meaning day, date, and month, is integrated without overwhelming the layout. The moonphase sits at six o'clock, which is where it belongs on a watch like this.

The double box sapphire crystal setup, one on the dial side and one on the caseback, is the detail that gives the Duobox its name and its signature look. It adds a little domed presence on the wrist that feels period-correct without being a costume.

The Movement

Brellum uses a COSC-certified movement inside, keeping the chronometer standard that the brand has maintained across the Duobox line. COSC certification means the movement is tested to -4/+6 seconds per day accuracy across a range of positions and temperatures. For a watch with this many complications, maintaining that standard matters.

The caseback sapphire crystal means you get a clear view of the movement, which is part of the appeal of buying into the Duobox concept in the first place.

Who This Watch Is For

You probably already have a sense of whether this is your kind of watch. If you're drawn to vintage-flavored complications, smaller independent brands, and watches that punch above their weight technically, the Duobox 39 TC Moonphase is squarely aimed at you.

• Collectors who want a full-calendar moonphase chronograph without going to a major maison
• People who find 42mm-plus cases uncomfortable but still want serious horological content
• Anyone who appreciates retro chronograph aesthetics done with actual Swiss watchmaking credentials behind them
• Buyers who care about COSC certification as a baseline quality signal
• Fans of small-batch production, where you're not going to see your watch on every wrist at a meetup

How It Compares

At 39.5mm, there's genuinely not much competition in the full-calendar moonphase chronograph category from established names. Most go bigger. Some go much more expensive. Brellum sits in an interesting gap: more complicated than entry-level indie offerings, more accessible than grand complications from houses with century-long legacies, and sized for people who actually think 39mm-ish is the right number.

The piston-style pushers are a nice detail that separates it visually from generic chronograph layouts. They look correct on a watch with this vintage brief. Muller clearly put thought into keeping the whole package coherent rather than just stacking complications onto a chassis.

Brellum keeps its drops small and deliberate. The blue Duobox 39 Triple Calendar Moonphase Chronometer fits that pattern. If you've been watching the Duobox lineup and waiting for the right version, blue might be it.