People writing letters on walls:
sometimes for art, sometimes for the joy of color,
sometimes for other reasons: gang messages,
warnings to stay away. At their best,
the letters on walls change a gray day
into the atmosphere of a sunrise. This is type both
very visible and invisible: the writing is front and center
but the letters may vanish into a design
that has a life of its own.

From Pittsburgh's Eliza Furance Trail