During the period 1990-1999, the United States Postal Service released millions of solo definitive stamps with forty-five primary designs, not including design differences of color and other varieties. This is more than three times the number of primary designs of regular definitive stamps from the 1980s. Many of the most widely used solo definitive stamps of the 1990s included a depiction of the American flag as part of the primary design. A few of the American flag definitive stamps of this period include the 29-cent Flag Over Mt. Rushmore, 29-cent Flag Over White House, Old Glory G-Rate, and 32-cent Flag Over Porch. Dozens of color variations and/or design changes in these particular definitive stamps are considered important enough varieties of the primary issue's design that they are included in major reference materials of United States stamps.
The USPS also issued a large number of higher denomination definitive stamps during this period with more regularity than any previous decade. These issues include the 2.90-dollar Eagle, 2.90-dollar Futuristic Space Shuttle, 5-dollar Washington and Jackson and the 11.75-dollar Piggyback Space Shuttle stamp issues.