Nearly every station in the London Underground contains an enamel plaque depicting a labyrinth. The collection were installed in 2013 by artist Mark Wallinger to mark the system’s 150th anniversary. Each of the 270 black and white designs is unique to its location, and all of them are posted in publicly accessible locations, so visitors can examine them directly, tracing the path with a finger. They’re numbered according to the route taken by the contestants in a 2009 Guinness World Records challenge to visit all stations in the system in the fastest time.

A list of all 270 labyrinths is here.

Graffito, Paris, 2013.

Below: In 2008, while anti-graffiti vigilante Fred Radtke was overpainting New Orleans street art with featureless gray, Banksy visited the city to leave an image of his own.

When E flat made its entrée into the drawing-room, C and G considered it a third person.

‘It’s a dominant,’ thought A flat, while E natural cried out, ‘I recognize it: it’s my leading tone.’

… But the same holds here as in music where the chord of G sharp has not the same meaning, depending on whether you reach it by way of the sharps or of the flats, and does not sound the same as that of A flat to the sensitive ear, though composed of the same notes.

— André Gide, journal, January 14, 1912

In 1991, artist Michael Dennis installed his sculpture Reclining Figure in Vancouver’s Guelph Park.

In 2012, prankster Viktor Briestensky erected the sign below at the park’s southwest corner.

Park staff initially removed the sign, but when a petition gathered 1,800 signatures they replaced it in 2014. The city now recognizes it as an official public art installation.

Two sets of reversible faces by Rex Whistler, from 1930 — above, Mayor & Judge; below, Scotsman & Englishman.

In 1590 Emperor Rudolf II commissioned Flemish painter Joris Hoefnagel to illuminate the Mira calligraphiae monumenta, an illustration of various scripts that had been begun 15 years earlier by court calligrapher Georg Bocskay.

The book contains a tiny demonstration of Hoefnagel’s skill in trompe-l’œil. On one page he painted a Maltese cross, a type of flower, depicting it as though the plant’s stem passes through a slit in the paper. On the overleaf he continues the idea — the foregoing text and images can be discerned through the page, and Hoefnagel has faithfully painted in the flower’s “stem” as if the insertion were real. It’s the only painted element on this side of the page.

British artist Martin Creed introduced a controversial installation in 1995: an empty room in which the lights turn on and off at 5-second intervals.

Critic David Lee said, “Last year, the Tate was scraping the barrel. This year they are scraping the scrapings … A light being switched on and off is not a good work of art.” But when Creed submitted the work for the Turner Prize, the jury praised its “strength, rigour, wit and sensitivity to the site.”

Work No. 227: The lights going on and off was followed in 2000 by Work No. 254: The lights in a building going on and off — in which a building’s lights go on or off each second.