The word “graffiti” usually conjures images of people with spray cans illegally making murals or jotting down tags using colorful paints. A lot artistic interventions use other tools and materials, though, subverting expectations and working in (literal and legal) gray areas to create works without leaving a conventional trace. Consider, for instance, the massive deep sea monsters, jungle predators and swamp creatures of Russian illustrator Nikita Golubev that lurk in the grimy shadows on the sides and backs of trucks.

Along similar lines, this series of skulls by artist Orion was made by scrubbing car exhaust from an active tunnel. For those looking to deter street art and artists, subtractive interventions like these can be tricky to pin down. After all, Golubev and Orion are simply cleaning vehicles or public surfaces, albeit very selectively. In many cases, the end result is actually further cleaning — art like this often pushes municipalities to send out teams that then wash off entire areas to make them look consistent again.

This artistic approach draws different reactions depending on the scale and situation. Take snow calligraphy, for instance — few people seem to mind a nice message traced into the hood of their car. Artist Faust notes that virtually “everyone has an affinity for writing in the snow as a child,” so perhaps it’s also something people feel they can relate to on a more experiential level.

Reverse graffiti not only provokes different response—it also spans a variety of materials and methods and can work with greenery as well as it does with snow or grime. Some moss artists work additively, creating mixtures to apply to surfaces and thus encouraging moss to follow particular grown patterns to produce an specific result. Others, however, actively remove moss to create desired words, patterns and illustrations.

Stefaan de Croock falls into the latter category, using power-washing tools to create cityscapes and other whimsical scenes on surfaces previously covered in layers of moss. As with a lot of reverse graffiti projects, his pieces are generally temporary — the moss simply grows back in to fill the voids over time.

SpY’s work in Besancon, France, combines elements of reverse graffiti and tree sculpting. Turning topiary approaches into a mural-making technique, he shaped vines into a circular work of wall art using an elevated work platform, trimming his way toward a perfect circle.

Light art and projection bombing are even more temporary and generally even less invasive than reverse graffiti. Lighting machines aimed at buildings to create patterns or spell out messages can be targeted and disabled if they persist, but ephemeral light painting like the above work by Lichtfaktor are deployed quickly using glowsticks or other portable devices and have to be captured on camera to work, making them brilliantly elusive.

Meanwhile, if light art is about brief visibility, then hydrophobic spray art is about lasting invisibility. Both are made to be seen and not seen in particular ways, but the latter has a key ingredient that determines when a design or artwork is visible: wetness.

When the hydrophobic NeverWet spray came out, it promised to waterproof everything, but some users found they had mixed results in applying it to things like clothing and touchscreens – it discolored shoes and left a sticky film on devices. Then someone thought to create a stencil and tag sidewalks with the stuff and a new type of visible-when-wet graffiti was born. One can, of course, free-spray with it as well, but stencils help make the outcome more controlled and predictable.

The same basic artistic idea has been applied in other contexts by designers, too — selective hydrophobia can be incorporated into brick pavers or concrete sidewalks, for instance, to create patterns that change with the weather. Clever urbanists might consider ways to integrate useful messages or wayfinding elements into such projects, like arrows pointing to nearest sources of shelter in a storm. Similarly, messages that appear on surfaces that change color with temperature could be used to guide people on particularly hot or cold days.

Practical designs aside, artists continue to experiment outside of traditional material palettes with works that raise questions about visibility and invisibility. There is fun to be found at the intersections of these ideas, embodied in projects like this one by 3D artist Milane Ramsi, who combined different approaches into a single installation — a concrete pillar appears to vanish while simultaneously revealing three-dimensional lettering more reminiscent of conventional graffiti.

Additive or subtractive, vandalism has its grey areas, but what about the seemingly more straightforward removal of graffiti? Here again there are shades of gray. Some artists, like Mathieu Tremblin, paint a surface clean then write over graffiti with their own work — in his case: humorously replacing loose tags with digital-style tag clouds. That, clearly, is of a kind with what was underneath; in places where tagging is illegal, replacement tags are too.

Then there is the saga of the infamous Banksy and the so-called Gray Ghost, an anti-street artist who leaves signature splotches of gray in the wake of his graffiti removals. Some argue the Ghost’s work vandalizes art — others say it is itself art. Legally, like Banksy’s illegal murals, both artists work in a similar space, though in Banksy’s case building owners often go to great lengths to preserve his art, in part for its monetary value.

Exceptions aside, municipal laws are usually clear on painted public art and the goal is generally total graffiti erasure, though it doesn’t always work out that way. This kind of official cleanup usually draws binary responses, viewed by some as a welcome fix and by others as an act of defacement. One award-winning film, however, argues for a third point of view: graffiti removal as the ultimate next step in the progression of modern art. Despite its semi-satirical intent, The Subconscious Art of Graffiti Removal raises provocative questions about what constitutes street artwork in the gray areas of additive and subtractive graffiti.

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.

Rainworks are pieces of street art that only appear when wet, featuring messages, images and interactive games that work great for a city infamous for its frequent precipitation. The idea, in part, is to encourage people to enjoy the rain, and reward those who go out and play in the gathering drizzle.

Peregrine Church creates these works which, while temporary, can last for months before degrading – the water-repellent sprays used are eco-friendly and will biodegrade when the designs eventually dissolve.

Neverwet and its analogs are not exactly new materials, nor is the idea of applying it to stenciled graffiti art novel, but the variety of Church’s work is compelling, as is his targeting of its results to different neighborhoods in an oft-soaked city, from the U District to Ballard and beyond. Some of the vignettes are simply playful images, or geek and gaming references, while others carry water usage-related messages.

Indeed, this seems the perfect set of urban settings – while Seattle has frequent rains it rarely has downpours, meaning the ground is often moist but the weather rarely so bad that it is difficult to go out and experience it.

For better and worse, self-reflection and meta-art is an inevitable facet of the digital age – this artist recognizes the blurred lines between physical walls and their Facebook equivalents, commenting on our relationship to social media while making light of it as well.

iHeart(StreetArt) is a street artist based in Vancouver whose creations span the spectrum from traditional tags and stencils to this series of works reflecting on our virtual lives and interactions, particularly how young people are raised in this strange new culture of communication.

He variously uses, abuses, twists and repurposes elements of our wireless language, giving hashtags and message bubbles new meaning. While he is not alone in this pursuit of geek graffiti or in using street art for social commentary, his dark humor and unique style set him apart.

This is also only a smaller sampling of a much larger body of work by iHeart which spans various street art styles as well as gallery and installation works on topics beyond contemporary digital interaction.

For years, this graffiti calligrapher has made his mark around New York City, but some of his best work is reserved for special (and somewhat unpredictable) occasions, relying on seasonal snowfall.

Known for his interior and exterior calligraphic murals as well as his signature snow pieces, Faust notes that virtually “everyone has an affinity for writing in the snow as a child. When most people grow up they lose the urge. I guess I just haven’t been able to shake it.”

As with other forms of reverse graffiti, his snowy script is ultimately temporary and removes material that would be wiped and swiped away by brushes or wipers, thus leaving no lasting impact or damage.

Some of his messages speak to the season (of snows and ice) while others are simply signatures left on the hood, trunk, windshield or window of a car as a simple surprise to brighten the dark winter days of passers by.