To earn the right to operate one of London’s iconic black cabs, aspiring drivers must pass one of the hardest tests in the world, known as “The Knowledge.” It requires knowing central London’s maze of 25,000 streets in intimate detail. In oral exams, the applicant must recite an efficient route between any two points the examiner chooses. These can be drawn from more than 125,000 points of interest, from obvious landmarks like Buckingham Palace to an obscure back-alley pub. Those who pass the test study for an average of four years.

Not surprisingly, a good map is an essential study aid, and the map above is an example of the type many students use. It’s made by Geographers' A-Z Map Company, commonly known as A-Z (pronounced by the British as “A to Zed”)—an 81-year-old map publisher with a history as colorful as its maps.

“The A-Z maps are very detailed, and you need that detail for The Knowledge,” says Peter Allen, a London taxi driver and a co-owner and teacher at Knowledge Point School, a school that tutors aspiring drivers. “They’re also very clear.”

View Images A page from an A-Z atlas shows the area around the British Museum (the big red building near the center), with taxi routes highlighted in blue. Map by Peter Allen, Knowledge Point School

For instance, Allen says, taxi students sometimes refer to major and minor roadways as “oranges” and “lemons”—a nod to the distinctive orange and yellow color scheme used by A-Z to indicate the hierarchy of roads. Smaller streets are white, which allows students to mark up the maps in their A-Z street atlas with their own favored shortcuts (see above).

But A-Z maps aren’t just for taxi drivers. They’re also used by tourists—and even London residents who need a little help navigating the city’s bewildering network of twisting streets that seem to change their name or direction of travel from one block to the next. The maps pack a remarkable amount of information into a small space without sacrificing legibility.

Originally this was all done by hand, as you can see if you look very closely at the map below (a detail from an A-Z map of Birmingham). “Lwr. Tower St.,” near the center, was written by hand and appears in a slightly different font. “This is … a piece of cartographic history indicating how these maps were originally made,” says Simon Kettle, a cartographer at A-Z.

View Images This map of Birmingham contains a hand-drawn relic, Lwr. Tower St., near the center. Maps reproduced by permission of Geographers’ A-Z Map Co Ltd.

The company was started in 1936 by a woman named Phyllis Pearsall, the daughter of a Hungarian cartographer who’d emigrated to London at the turn of the century. By her own telling, Pearsall got lost going to a party one rainy night using an outdated map from the Ordnance Survey, the British national mapping agency. Determined to create a better map, she walked the streets of London by herself, logging more than 3,000 miles and working 18 hours a day to produce her first atlas.

Charming as it is, the story has its skeptics. They include Pearsall’s half-brother, who created a blog dedicated to debunking it, and Peter Barber, the head of maps at the British Library, who has argued it’s more likely that Pearsall updated and improved upon other street maps already available at the time—such as the similar-looking ones her father made.

Whether or not Pearsall’s story is a true account or a clever bit of marketing, the company she started has grown into the largest independent map publisher in the United Kingdom, with more than 350 maps and atlases that span the country.

View Images This section of the A-Z map of London shows the area around St. Paul’s Cathedral (top left) and London Bridge (bottom right). Map reproduced by permission of Geographers’ A-Z Map Co Ltd.

Recently A-Z has been working with Uber, which faces new regulations in London. Last year, the city began requiring that drivers for Uber and other private-hire services have to pass a new “topographical assessment.” It’s a far cry from The Knowledge, but private-hire drivers now have to demonstrate basic geographical proficiency, such as being able to say which major roadway leads to a specific airport or nearby town. They’ll also need to demonstrate that they can use an A-Z atlas to find a particular street or landmark, and to plan a route between two places.

Kettle says the company is supplying atlases to Uber for training and testing its drivers. (As with the black cabs, the city regulations mention A-Z maps by name, but drivers are free to prepare using other maps if they wish.)

How long paper atlases can persist in the digital age is an open question. But for now, at least, they seem to have a place in helping taxi drivers master The Knowledge—and perhaps in helping Uber drivers figure out which road goes to Heathrow.