RCA introduced the 45 rpm single record in 1949 as a smaller, more durable, and higher fidelity replacement for earlier 78 rpm disks. Music singles on 45s served for decades as a low-cost entry point for young fans and turned single record sales into a prominent gauge of the popularity of songs and musicians. The durability of the 45 rpm record also made it useful as a product for children, either on its own or as a companion to a book.