People have been scribbling on walls as long as they have been building them. "Graffiti"—the word comes from the Italian verb graffiare, "to scratch"—covers a wide range of public inscriptions, from the early paintings on the walls of caves at Lescaux to quips hastily inked up on contemporary bathroom stalls. The late-twentieth century has seen the development of a market for graffiti as an art form, although the majority of graffiti remains unsolicited and anonymous.

Historically, graffiti has been used primarily as a form of personal communication. One of the earliest uses developed in the United States among hobos who rode the rails across the country in the first decades of the twentieth century. The complicated symbolic language of these transients was scratched in chalk on fence posts and other unobtrusive spots to communicate the receptivity of the towns-people to future travelers.

One of the most...