LONDON — As the world's oldest subway, better known as the Tube, celebrates its 150th birthday, here's a familiar but gently tweaked reminder:
Mind the map.
The London Underground is justly famous as a defining feature of the British capital, a wonder of the modern age that whooshes millions of riders around the city every day.
But a London institution that may have an even tighter grip on the public imagination is having a birthday too (its 80th): the Tube map.
Instantly recognizable the world over, the simple yet elegant diagram of the 249-mile subway network is hailed as one of the great images of the 20th century, a marvel of graphic design. Its rainbow palette, clean angles and pleasing if slightly old-fashioned font (Johnston, for typography buffs) have endured since hurried passengers first stuffed pocket versions of the map into their raincoats in 1933.
"It's a design icon," said Anna Renton, senior curator at the London Transport Museum. "You shouldn't use that word too often, but it really is."
It's been copied by other cities and riffed on by artists and satirists. It's omnipresent in souvenir shops, plastered on mugs, underwear, mouse pads and tote bags, on sale next to the "Mind the Gap" T-shirts.
Perhaps most impressively, the image is stamped onto Londoners' brains. If the Tube is how people get around London, the Tube map is how many conceive of this sprawling city, their sense of its geography shaped — and sometimes warped — by the drawing's streamlined, reductive layout.
Tell a Londoner the name of a neighborhood on the other side of town, and you may get a blank stare; mention the closest Underground line and station, and the mental GPS kicks in.
All credit goes to a problem-solving electrical draftsman named Harry Beck, who came up with the design in his spare time in 1931.
At that point, more geographically accurate maps of the Tube were already in use. But the expansion of the network into the suburbs made such depictions unwieldy, and their curving, intersecting lines had come to resemble a nest of snakes, a plate of spaghetti or London having a very bad hair day.
Inspired, some say, by electric-circuit diagrams, Beck straightened out the lines, drew only 45- and 90-degree angles, and truncated distances between outlying stations. Then he submitted his unusual schematic rendering to the London Underground's publicity department.
Officials rejected it.
"They thought it was too radical, and they thought that people maybe wouldn't be able to understand it," Renton said.
Undeterred, Beck made a few minor adjustments and tried again. This time, officials decided to give it a chance, printing 750,000 free pocket-sized copies in January 1933, with an almost apologetic explanation attached: "A new design for an old map. We should welcome your comments. Please write to the publicity manager."
No responses have been unearthed yet in the archives, said curator Renton. "We can only assume it was widely accepted and really taken on board by Londoners. And they never looked back."
The design led to imitations around the world. Within a few years, it was copied by the transit system in Sydney, Australia. The New York subway map of the 1970s also paid homage to Beck's brainchild.
"It was absolutely revolutionary. I always say it's probably the best bit of information design of the 20th century," said London native Mark Noad. "It was his ability to simplify it and still make it understandable that was his great achievement."
Noad should know: A designer himself, he got a taste of what it's like to tamper with a classic when he had the temerity to produce his own version of a Tube map two years ago, just for the fun and challenge of it.
Over the years, Noad says, London Underground has updated Beck's original diagram but not necessarily for the better; in his opinion, new train lines were drawn onto the map in a way that skewed geography further and made things more confusing.
In his reworking, Noad replaced the trademark 45-degree angles with more sinuous 30- and 60-degree angles and tried to make the lines more closely mirror London at street level. He even commissioned a new font, "New Underground Condensed," that's better for squeezing in the names of so many more stations — there are now 270 — than existed in Beck's day.
Although some people complimented him on his effort, others called Noad's map disorienting and berated him for trying to fix something that wasn't broken.
"I didn't really explore this whole subculture we have of people who are obsessed with the map. I've come across quite a few of them since," Noad said, then paused. "I've had very interesting conversations."
Even London Underground makes changes at its peril. When it erased the Thames from the Tube map in 2009 in order to de-clutter the drawing, there was a public outcry and a "Bring back the Thames!" campaign. Officials restored the river a few months later.
Whether that feature actually helps people navigate London is another question. The biggest criticism of today's Tube map is that it's so out of kilter with above-ground London that it misleads passengers as to where things are truly located and their distances.
For example, tourists (and even some longtime residents) might think it's a time-saver to take the Tube from Leicester Square to Covent Garden in the theater district; after all, they're a stop apart on the Piccadilly Line. But walking takes less time.
By contrast, two outlying stations that appear the same distance apart on the map as Leicester Square and Covent Garden could be miles from each other.
As distorted as it is, the map still offers a strong sense of London's layout, a benefit not to be underestimated given how confounding the city's hodgepodge of streets and neighborhoods can be.
"London's actually a very difficult city to get your head around…. I'd travel to central London by bus, and I'd actually see the streets, but ask me to draw a map and I couldn't do it," said Max Roberts, a psychology professor at the University of Essex who has studied the effects of maps.
"The Underground map does help people to conceive of London," he said. "It does distort London, but without it, they'd have no idea of London [geography] at all."
Without the map, which is still free, the London Underground would also be poorer. Tube-related merchandising and licensing fees bring in about $7.5 million a year, said Mike Walton, head of retailing at the London Transport Museum. That includes the use of non-map-related items, such as the distinctive Underground symbol of a red circle with a blue bar running through it.
But "of all the signs and logos," Walton said, "the Tube map is kingpin."
Beck's payment from London Underground for his pathbreaking design: 5 guineas, or about $600 today.
Before his death in 1974, he complained less about the money than about losing control over the map and sometimes being stinted credit as its creator. But he lived to see it become almost as indelibly identified with London as Big Ben or Buckingham Palace.
In 2006, in a contest sponsored by the BBC, Britons named the Tube map as their second-most-loved British design of the last century. The top vote-getter was the supersonic Concorde jet.
But which is the real winner? The Concorde was mothballed in 2003. Ten years later, and 80 years after it was first distributed free to busy commuters, Beck's map lives on, indispensable as ever.
Special correspondent Janet Stobart contributed to this report.