Most cities have so much in common that a generic “map of every city” can seem similarly familiar to people living in London, Paris, New York or another metropolis entirely. General types of neighborhoods aren’t the only things different cities share, though — much less obvious but pervasive are sets of codes, symbols and markings that can communicate meaning across different times and urban spaces.

Even though (or perhaps because) people drive over and walk by them every day, it is easy to overlook the rich, colorful and cryptic utility markings spray-painted onto streets and sidewalks. Like graffiti tags or hobo codes, this language of scribbled text, dots, lines and arrows may seem indecipherable at first, but lives depend on engineers, city workers and utility companies understanding what they mean.

Utility markings tell excavators working on subsurface projects where to dig and (more importantly) where not to dig. A vocabulary of symbols (with its associated grammar of colors) helps diggers steer clear of dangerous power, sewer and water lines as well as other pipes and cables.

Like any language, utility codes have evolved what one could call regional “accents” of a sort — linguistic conventions that vary from one state or country to the next. Standardization, though, is important in helping keep people safe, which is why there are often local or national rules governing what different colors and symbols represent.

Long before cities came around to the idea of utility markings, train-hopping nomads were working out similarly symbol-based systems of communication. As these travelers roamed America looking for work, particularly during the Great Depression, they learned to leave messages for one another — so-called “hobo codes.” These relatively simple symbols could help fellow travelers find good places to camp and kind people who might give them meals, for instance.

At the time, being nomadic was a mixed bag — some travelers were known as bums or tramps, disparaged for drinking or idling rather than working. The term hobo, though, was more specifically applied to those actively seeking work and willing to take on jobs others didn’t want to do — hobos were met with various degrees of caution and generosity. Many were illiterate, however, so coded symbols with intuitive meanings helped them convey messages through etched or chalked markings. The relatively discreet size and abstract shapes made these marks easy for people not in the know to overlook.

Some symbols represented fairly specific suggestions about how to behave and what to avoid. A cross, for instance, could indicate that talking about religion might help a person get free food from a particular resident. Other markings might caution hobos about heightened crackdowns on vagrants and beggars by local police. While train-jumping culture has changed, some modern travelers have attempted to digitize the idea of hobo symbols through QR codes.

Mural and graffiti art sit somewhere between officially sanctioned and illicit urban communication, depending on the location and surface being tagged. These interventions, too, have evolved a lot over the years. Definitions and genres have sprung up along the way, helpful for tracking and analyzing but also understanding different works — there are pieces, tags, stickers (or: slaps), throw-ups, stencils, heavens, blockbusters, wildstyles and more.

A piece, for instance (short for “masterpiece”) and is usually a complex and multicolored affair difficult to do illegally given the time they take to make. A blockbuster can go either way, often made using rollers and designed to cover up a surface — sometimes one that has already been tagged. A heaven, however, is generally illegal, defined by the difficulty of putting a work up high on something like the back (or front) of a highway sign or the surface of a billboard advertisement — not generally places where one can get official approval to paint.

Some typologies are pretty self-explanatory, like bubble letters or fat caps, the latter of which are usually done with wide spray tips, making them both easy to deploy and easy to read (in turn rendering them useful for get-in-and-out-quickly situations). Shadow letters can also take a bit more work, but help a tag pop off a surface, giving it a somewhat more weighty and three-dimensional appearance. Indirectly, the forms and shapes of letters and symbols tell the observer something about the artist’s intent and constraints. Graffiti can even be broken down into characters, alphabets and fonts, which an informed onlooker can use to better understand a given work.

Some artists and art fans have gone to great lengths to classify different types of graffiti, but such a task is destined to be forever incomplete — graffiti is personal and location-specific, not based on any shared font or type. But some, like artist Evan Roth, try anyway to collect, identify and compare examples of letters, creating order out of the seeming chaos of conflicting tags. He also took his project full circle by pasting up alphabets along the city blocks in which they were originally found, encouraging people to look at tags in a new light, offering temporary glimpses into the linguistic ties that bind them loosely together. With any street communication, legal or illegal, there will always be some give and take between fluid creativity and efforts to categorize, standardize and simply understand.

Computers have a real problem when it comes generating truly random numbers, which has led one web-critical cybersecurity firm to reference an array of lava lamps to create unique and unpredictable code.

The lamps are recorded on video in Cloudflare’s San Francisco headquarters, then translate those movements to create encryption keys for the web. The results help protect internet giants including Uber, OKCupid, FitBit and others, totaling around 10% of global traffic.

As Tom Scott reports, Cloudflare has good reason not to trust algorithm-generated code, relying instead on the effectively erratic movements in groovy retro fixtures set on shelves instead. Anything made using algorithms could potentially be guessed at or reverse-engineered by sophisticated hackers — fluid dynamics of lava lamps, not so much.

And these top-secret cryptographic devices are not hidden behind lock and key, but publicly on view. In fact, additional disturbances from human proximity can add more randomness to the equation. In the end, it’s a surprisingly low-tech solution, but it mixes things up and ultimately does the job.

If you’re among the attendees of international technology festival Campus Party, which will take place during the first week of September in London, you have a much better chance of cracking the codes featured in a new series of street art than the average passerby. The murals were commissioned by the festival in celebration of the 75th anniversary of the Government Code and Cypher Schools team, which broke the Enigma code.

Each freehand painting features the face of a code developer, including Alan Turing, Samuel Morse and Tim Berners-Lee, along with a code. Onlookers are challenged to break the code, and anyone who successfully does so can enter it into a special location on the Campus Party website for a chance to win two tickets to the event.

“Mixing raw graffiti with complex binary decimals, these visual contradictions are set to spring up in London, Manchester and Birmingham as we work our way towards the launch of Campus Party at The O2 on September 2nd,” say the organizers.

Other forms of code that have popped up in street art include QR codes for digital nomads, binary and other ‘geekfiti‘, and, of course, traditional ‘hoboglyphs.’

Over six years, one dedicated man has constructed his dream house, complete with (presumably fake) boulders, trees, gardens, pools and ponds. It is the kind of luxury home (or man cave) you might expect to set into the side of an actual mountain … rather than perched atop an urban apartment building.

The Daily Mail  reports that Chinese eccentric Zhang Lin slowly assembled this structure in Beijing despite disruptions and dangers caused to those residents below, apparently without planning permission and in defiance of zoning laws (not to mention common sense). The many tons of material required were hauled up the building, then transformed into a faux mountaintop. In turn, the industrious top-floor owner tucked his unique private penthouse retreat into the resulting rock face.

A number of residents have complained about the noise from construction, and one has experienced flooding, either from fake mountain pools and ponds above their place, broken drainage due to displaced infrastructure, or perhaps both.

Meanwhile, the penthouse – originally slated to be a small shack added to the top – has sprawled in all kinds of creative and (to those below, at least) unexpected ways. There are various outdoor decks, viewing platforms and winding paths that make their way through and around the stone mountaintop and its outgrowths of greenery.

And what comes next? Well, no one in the city has stopped the project so far, but as complaints mount from other residents, and as the weight loaded on the roof grows, one has to wonder what will happen first: government intervention or a tragic structural collapse. Update: the government has now stepped in to demand the demolition of this penthouse, which must now be carefully deconstructed, piece by piece.

If you are going to have to wait for a ride anyway, why not do some shopping? No, not on your laptop or that tiny screen of your smartphone – right there on the wall, via virtual aisles on a life-scaled billboard full of products.

First in South Korea thanks to Tesco, and now popping up in Australia and other countries, it works something like this: a range of popular everyday products is displayed on the wall; scan the QR codes of the ones you want and your purchase will be added to a digital shopping cart.

In many cases, your purchases will arrive the next day – in some, they will be waiting for you when you get home, saving you that side trip to the corner store or local supermarket.

If nothing else, this presents an interesting technology-bridging mechanism for those reluctant to shop online. They choose from real products, presented at actual size, in a potentially more familiar mode of browsing than via the internet.