666, in the collection of the Yale University Art Gallery, is an example of the frottage postcards that Indiana sent to his friends in the 1960s. “666” first appeared in Indiana’s painting USA 666 (The Sixth American Dream) (1964–66). The artist explained that it came from multiple sources, the 6 of his father’s birth month, the Phillips 66 sign of the company his father worked for, and Route 66, down which his father disappeared when he left his family.
Indiana sent this card to Richard Brown Baker, a collector who interviewed Indiana in his studio in 1963, on June 6, 1966, with a note saying “Once in a lifetime. More 6s for you.” The artist sent similar cards to Alfred Schmela, his German dealer, and to the composer Virgil Thomson.