As hip-hop spread, so did graffiti. The book Subway Art, which documented the elaborate designs on New York subways, became a bible to graffiti writers. And graffiti's popularity spread to Europe, particularly France and Germany. The style, and sometimes the demographics, of graffiti writers differ from each city. Los Angeles writers use walls to create murals in the tradition of the Mexican mural heritage.
Brazil uses tar because it's affordable.
Boston's different because most of its writers hail from the suburbs. No one knows exactly why. Other American cities have both inner-city and suburban writers. Allston Village executive director Jennifer Rose thinks she knows the answer. Rose's organization, Allston Village, works with local businesses in maintaining storefront upkeep and providing technical assistance.