A watch's primary function is to tell us what time it is. This is why telling the time is not considered a "complication".
But anything beyond that is.
And some complications are much more difficult to make than others. And combining more than one of these complications into one watch is absolutely spectacular, hence, it's own category: Grand Complications (minimum 3 complications to qualify).
But even doing one of these things well is a challenge, and making them all, even in individual watches make for quite the repertoire.
Traditionally speaking, there are 6 'Master Complications', and they are:
1. The Ultra Slim - this simply refers to a movement and the watch that houses i that is ultra slim.
2. Moonphase Calendar - As the name suggests, a moonphase calendar will not only indicate the date, but the phase of the moon as well, and is decorated with what is (usually) a very nice graphic of the moon and sun that rotates.
3. Perpetual Calendar - A perpetual calendar is a spectacular complication. You're not a true top-10 brand unless you've made at least one. This complication will not only tell it's wearer the time, day, date, month, year (leap years included), and moonphase, but it will do so in perpetuity changing for example, from the 30th to the 1st when it's a 30-day month, and even from the 28th to the 1st when switching to March in a Leap year. This is where "perpetual" gets its true meaning - in other words, not the way Rolex uses is to describe a basic automatic movement.
4. Tourbillon - The tourbillon was invented back around 1795 and refers to an addition to a watch's escapement designed to counter the effects of gravity. This is done by mounting the escapement and balance wheel in a rotating cage. By today's standards, other more efficient ways have been developed to do the exact same thing, but none look as spectacular (or are as technically demanding) than a well made Tourbillon.
5. Split Chronograph - A split Chronograph (rattrapante or Doppelchronograph), is a chronograph module that has two hands that 'split' so that the wearer can time intervals. It's not an easy thing to do well, and as such, few have attempted it. But again, a sign of true technical prowess.
6. Minute Repeater - not an alarm, but the first complication to use sound, a minute repeater exists for the sole purpose to announce the time using chimes. All minute repeaters tend to be great, but the best of the best will have nice and clear chimes and patterns. This was obviously more practical back when luminescent hands were not yet invented, but today, it's pure elegance and easily worthy of being considered a 'Master Complication'.
Blancpain, when they were relevant and still a truly high-end brand, started the 80's on a mission to create one of each, and by the time the decade was over, they achieved all six in the exact same 33mm case.
And while the case is considered small by today's standards, the point is stuffing all that tech into such a small space is without a doubt, 'walking the walk'.
Of course it needn't be said that the obvious players like Patek Philippe, Audemars Piguet, Vacheron Constantin, Jaeger LeCoultre, A. Lange & Sohne, have all achieved this remarkable feat.
And i've just mentioned that Blancpain did it as well. But who else has and maybe more interestingly enough, who hasn't?
IWC has all of the Master Complications in it's current repertoire except for a true Ultra Slim.
The same can be said of Zenith: the Ultra Slim isn't as "impressive" to the masses as it used to be (it should be, though), so it's the first to be dropped.
Glashutte Original has at least 4 of the 6, missing a true Ultra Slim (although they do have some reasonably slim watches), and a minute repeater.
Omega's movements are no longer made by Omega (they're made for them by ETA), but they'll gladly sell you a split chrono, tourbillon, and moonphase, but no perpetual calendar, minute repeater, or true ultra slim.
Even the all-popular Rolex has made exactly ZERO of these complications.
At best, and its a stretch, one could argue the Cellini line has some "slim" watches, but the Cellini line is not the go-to collection when someone thinks "Rolex".
Rolex once made a watch that housed a Valjoux 55 VBR split chronograph back in the 40's (ref. 4113), and also made a moonphase from that same era (ref. 6062 and it's sister watch the 8171), but these used movements based on the Harwood Automatics of the era with very very low production (around 50 combined).
And thats it!
The point is, making any of these complications puts you in elite company.
So elite, very few companies have ever made any of these complications (in-house or otherwise), and even less still make them today, and less still can claim to make them all in-house today.
Even more impressive is when those that do, exceed all standards and take it up a notch with complications like celestial star charts, constant force, or Sidereal time.
How's that for food for thought the next time you want to think about who the best-of-the-best is or isn't?