Fast forward to this year: TAG Heuer introduced the new Carrera “Glassbox,” reference number CBS2210.FC6534. That’s 13 characters, compared to the simple four-digit reference numbers from the 1960s. And TAG Heuer is far from alone: modern Omega’s reference system is notoriously long with 14 digits. Breitling uses a 12-character system. While we commonly use six digits to refer to Rolex models, the full reference is longer: the steel Submariner Date is ref. M126610LN-0001. Same for Patek Philippe – while it’s one of the few brands that has stuck to a four-digit base reference, its full references are longer. The new white gold Nautilus is reference 5811/1G-001. Is your head spinning with numbers yet?

Over the past few decades, reference numbers have evolved from a few easy-to-remember digits to long and complicated strings that even Raymond Babbitt would struggle to recall. How it happened is a story of humans, computers, and how they interact with each other.

To tell the story of reference numbers is to tell the story of the wristwatch. After the wristwatch picked up in popularity with soldiers during World War I, manufacturers began to focus on production at scale.

“In 1923, Patek Philippe committed itself to the wristwatch,” Dr. Peter Friess writes in the new book Treasures From The Patek Philippe Museum Collection. “In short order, that became the backbone of its business.” At first, these wristwatches were one-off or custom creations that often looked a lot like the pocket watches from which they evolved. But soon, wristwatches took on their own aesthetic, with their own technical innovations to solve new problems presented by wrist-borne timekeepers.

For Patek, this began with its first serially produced and numbered wristwatch, the Calatrava Reference 96. Just two digits, the reference number was simple and clean, an exercise in branding as much as anything else. As serialized production of wristwatches with interchangeable parts became commonplace in Switzerland, people needed a quick and easy way to refer to these products when communicating with each other, across departments, or with suppliers. Enter the reference number.

Other watch brands started with similarly short reference numbers: Rolex, Omega, and Heuer all used four-digit reference numbers, and many other manufacturers settled on three or four digit references through the ’60s. There was some internal logic to each manufacturer’s system, even if a bit haphazard.

For example, vintage Heuer’s four-digit reference numbers worked as follows:

• First two digits: caliber
• Third digit: collection
• Fourth digit: case metal

To take the automatic Carrera 1158 as an example: 11 stood for the automatic caliber 11; 5 represented the Carrera collection, and 8 indicated a gold case. Letters could then be added to the suffix to represent the dial; for example, the most recognizable is the 1158CHN, which stood for the champagne dial and black (noir) subdials.

Like Heuer, many numbering systems from this era were formulated to quickly communicate the case material, caliber, and complication: Universal Geneve, Omega, and Breitling, among many others, implemented similar systems. Sometimes an additional character would be added after a period or a slash to communicate additional information.

Once you know any given manufacturer’s system, these reference numbers are “human readable,” an easy way for people to recognize and catalog products. For example, if you’re a Heuer employee, all you have to do is look for an N or S in the reference number and you’ll know whether it’s a black or silver dial, respectively. This reflected the fact that, while the Swiss watchmaking industry had industrialized and began to use modern machinery, evolving from an industry of craftspeople producing watch parts by hand, humans still played an important role in operating machinery, tracking parts and inventory, and coordinating the general business of manufacturing. A three- or four-digit reference number may not say everything you need to know about a watch (is that on a strap or a bracelet?) but it’s close enough, and with human hands coordinating the entire effort anyway, there was room to figure out the rest along the way.

“It’s also possible suppliers were driving many of these conventions at the time,” says Nicholas Biebuyck, Heritage Director of TAG Heuer. Remember that through the middle of the 20th century, many brands were relying on the same few key suppliers: Singer or Stern for dials, EPSA or Spillman for cases, and so on. Across manufacturers, reference numbers took on a similar format to communicate the same information. And sometimes, the same numbers even meant the same thing; for example, “8” was used to represent gold cases by both Heuer and Rolex during that time (take that gold Carrera 1158 and the gold Submariner 16808 or GMT-Master 16758, for example). As we’ll see, in the ’70s a number of brands even adopted reference numbers with a similar format. It seems plausible that manufacturers would have references motivated by a need to communicate with their many suppliers quickly and easily.

Meanwhile, there’s some evidence that international distributors hated the inconsistent and often illogical numbering systems of their Swiss overlords.

“None of the distributors adhered to the Breitling manufacturer’s numbering system,” says Breitling historian and collector Fred Mandelbaum. “Breitling USA – the brand’s most important distributor in the world – called the Navitimer 806 the 9113, while the U.K. had their own four-digit system.” He explained that every local Breitling distributor had its own solution for numbering models and inventory.

“Every brand tried to have logic in their numbering system but failed miserably,” Mandelbaum continued. “It just represents the total absurdity of the Swiss trying to come up with a numbering system.” For example, if you opened a Rolex catalog in the mid-’60s, you could’ve found the following references listed next to each other:

• 1002 – Oyster Perpetual
• 1003 – Oyster Perpetual with engine-turned bezel
• 1012 – Gold Oyster Perpetual
• 1013 – Gold Oyster Perpetual with engine-turned bezel
• 1016 – Explorer
• 1018 – 36mm Oyster Perpetual
• 1019 – Milgauss
• 1024 – Gold-plated Oyster Perpetual

Some of the numbering logic becomes clear (the last digit representing the bezel type, for example), but assigning references to any given model jumps around. This is how it was with most brands through the 1960s.

Omega was one of the first to adopt a new numbering system in 1962 when it switched to an eight-character alphanumeric system without changing its watches. For example, the same Speedmaster CK 2998-62 became the ST 105.002. Omega declined to comment for this article, but the logic of the reference systems it’s used over the years is well documented.

“In 1962, a new system of reference numbers, called MAPICS was introduced to classify products into groups,” explains Moonwatch Only, the Speedmaster tome. According to Moonwatch Only, Omega’s MAPICS system continued to develop after it was introduced, which makes it difficult to establish a reference table that’s 100 percent accurate.

By the ’70s, many brands started following Omega, thinking better of their reference systems and implementing new ones. Mandelbaum said that Breitling, which was using three-digit references, was simply running out of good reference numbers and moved to a four-digit system. In the late ’70s, Rolex began changing from four-digit to five-digit reference numbers. Like the systems of other brands, these simple digits communicated the model, bezel type, and case metal.

To understand why this change started to occur when it did, it’s important to zoom out and remember what was going on in the business world outside of the watch industry beginning in the early ’60s. To say it in three letters: IBM. After World War II, IBM was absolutely cranking on computer technology, and savvy industrialists started exploring applications for the computer in the business world. One such application was supply chain management. IBM helped to develop the first MRP system (material resource planning, the precursor to the modern ERP) in 1960. Soon after, IBM introduced its first database system for commercial use.

By 1972, a group of ex-IBM employees founded SAP, today the world’s largest ERP software provider. Since then, its software has grown increasingly robust, but every additional feature also adds another layer of complexity (starting to sound like reference numbers yet?), confounding generations of analysts and consultants. (For a brief and ultimately unsuccessful tenure, I was one of these confounded consultants working on projects with cryptic titles like “systems integration.”)

Like every business, the watch industry started adapting to the arrival of computers.

As computers and enterprise software became more widely used, reference numbers weren’t just a way for humans to understand and communicate with each other about products – they were a way for humans to communicate with computers about materials and inventory. The reference number evolved from a shorthand used on an invoice or a phone call with a supplier to becoming a line of code in a database that represented something more specific.

“Today, every brand has master resource planning, and when it comes to figuring out reference numbers, it comes down to what’s easy and flexible for the software,” TAG Heuer’s Biebuyk said. This transition began in the ’60s and ’70s as manufacturers began to use rudimentary computers and database systems. Reference numbers evolved from something developed with the illogical logic of humans to the irrefutable logic of computers, formulated in a way that made it easy for computers and databases to understand. Humans came second.

In 1988, Omega changed to its PIC system (product identity code), an eight-digit numbering system that added to the previous MAPICS system by coding the dial color and band material. Around 1990, Breitling switched to a 12-digit numbering system that gives COSC and caliber identification, case finish, as well as dial color and design.

Throughout the mid-’80s and early-’90s, the use of computers became widespread in all aspects of watchmaking and other industries. When I spoke with designer Marc Newson about Ikepod watches, he noted the mid-’80s as the time when he first learned CNC (computer numerically controlled) machining, the automated control of machining tools with a computer that enabled huge gains in productivity and accuracy. CNC, for example, enabled Newson to design the rubber straps that would go on to be used with the Apple Watch some 30 years later.

Similarly, collector John Goldberger has pinpointed 1985 or 1986 as the beginning of modern watchmaking, when “the computer had really taken over a large part of the manufacturing process.” Fewer brands focused on hand finishing of cases and movements and automated these processes to increase the number of watches produced.

It’s no coincidence that this is also the time when MRPs evolved into ERPs – the term ERP was coined in 1990. While those early MRPs focused only on the coordination of manufacturing activities, ERPs integrated all departments across an organization. By now, you probably know what that means: Longer reference numbers. By the ’90s, it wasn’t just the manufacturing parts of the business communicating with each other; they were also communicating with sales, marketing, accounting, and all of the other departments of an organization.

“With SAP’s ERP, there’s a need to codify every single component of a watch,” TAG Heuer’s Biebuyk said. “Take the Carrera Plasma – every single diamond on that watch has an individual part number.” Without this type of master planning, it’d be impossible to coordinate the dozens of departments and disparate activities across a large manufacturer like TAG Heuer. Long gone are the days of phoning up Jean-Claude down in the cases department to ask where all the casebacks for the 1158s are.

“It’s no longer people differentiating whether a white or a black dial should go inside of a box,” Breitling’s Mandelbaum said. “It transitioned to computerized stock-keeping, so we needed a precise numbering system for this.”

In 2007, Omega updated its references to the current 14-digit PIC system that enthusiasts love to loathe. But it’s far from alone: Oris and TAG Heuer use 13 characters; Breitling uses 12. IWC and Cartier use a seemingly sensible eight. While we know the five-digit shorthand for a few of the most popular Audemars Piguet references (or maybe it’s just the 16202), its reference numbers might actually be the longest at 17 characters.

It’s easy to feel a bit nostalgic for the days of shorter reference numbers. Today, vintage Rolex collectors can shout four-digit sequences back and forth at each other in what sounds a bit like speaking in tongues to the uninitiated. Nowadays, no one knows the modern reference number of a Carrera or Navitimer; even the base six-digit Rolex references can be difficult to remember.

“At five digits, we stop remembering model numbers,” Mandelbaum said. “We’ve lost the ability to clearly refer to specific reference numbers. Of course, now it’s logical, but we can’t talk about reference numbers in the way we used to.”

Mandelbaum’s right, but only to an extent. While saying “Rolex 1675” might feel like a specific watch, it can still conjure up a different image for different people. Sure, it’s a vintage GMT-Master, but is it steel or gold? Gilt or matte dial? What about the bezel? On a strap or bracelet? Some of these details have only become important post-hoc – it’s hard to imagine too many people cared about pointed or square crown guards until relatively recently – but that’s also the point. The short reference number is a relic of a time when manufacturers ordered parts from their suppliers and then mixed and matched components to see what might sell. For the most part, it was people working with other people. Throughout the second half of the 20th century, computers became more involved with every single link in the supply chain, and with that squeezed the inefficiencies, human element, and maybe, some of the charm out of watchmaking.

Today, every single link of a watch’s production is perfectly orchestrated and coded into computers, often planned years in advance – from those lab-grown diamonds in the TAG Heuer Plasma to the ad you’re served up on Instagram. And reference numbers help make it all happen.

But it doesn’t take a body of psychology research to tell me that it’s near impossible to remember a reference number with more digits than jewels in its movement. Most brands have submitted themselves to the computer takeover, with reference numbers that are too long and complex for their own employees to remember.

The middle ground is to try what Rolex, Patek, and AP have done: the base number remains similar to a reference that’s been in use for years – almost like a root – and the rest of the information is hidden in numbers that the general public doesn’t bother with. The modern Rolex six-digit system might test the limits of our mental capacity, but it’s aided by the fact that it’s based on decades-old references; I’m only able to remember the modern Explorer II reference number (226570) because it’s based off the same reference from the ’80s (16570).

Really, this approach to reference numbers is similar to the way these same brands approach the actual products: subtle changes, tweaks, or additions over the years, but with a core DNA that’s immediately recognizable over generations.

The history of the reference number is really the history of the wristwatch. At the beginning of the 20th century, machines and industrialization allowed for the serial production of the watch. Manufacturers began producing watches with interchangeable components and needed an easy way to reference these watches when speaking with each other or with suppliers. And the modern reference number was born.

Throughout the second half of the 20th century, computers and technology became more instrumental to the manufacturing and production of watches, ultimately displacing many of the human elements of watchmaking. In the ’60s, reference numbers became ways for humans to communicate with computers and databases about watch manufacturing. By the end of the 20th century, computers were ubiquitous in all aspects of watchmaking, enabling accuracy, efficiency, and production levels that would’ve been unthinkable a few decades prior.

Just like computers have changed watchmaking and everything else around us, they’ve also had their way with the reference number. And while we might bemoan the human elements we lost along the way, the computers keep on coming.

But the human element isn’t completely lost. For example, when Rexhep Rexhepi introduced his Chronomètre Contemporain, it had a simple reference number: “RRCC01.”