The "love" stamps, issued for use on special personal occasions such as birthdays, anniversaries, and when other special sentiments are being sent, have become one of the more popular types of regularly issued U.S. stamps. This was the first of the series, designed by leading pop artist Robert Indiana. Since the late 1960's, Indiana has expanded his "love" theme to a series of sculptures, some of them monumental in size. He also has designed sets and costumes for theatrical productions, most notably for the Santa Fe Opera's 1976 production of Virgil Thomson's The Mother of Us All, based on the life of Susan B. Anthony.