"PRO" refers to performing rights organizations (sometimes called performing rights societies). The major performing rights societies in the U.S. are ASCAP (standing for American Society of Composers, Authors and Publishers), BMI (Broadcast Music, Incorporated), and SESAC (which originally stood for Society of European Stage Authors and Composers but now stands for “SESAC”). Of the three, ASCAP and BMI are by far the largest, as SESAC only has about 10% of the U.S. performing rights. ASCAP and BMI are non-profit, meaning they collect money, pay their expenses, and distribute everything else to the writers and publishers.