The term "album" in pop music comes from the early 20th century when music was sold on 10-inch and 12-inch discs. These discs were sold in a set of two or three and were packaged in book-like bindings, resembling a photo album. In the 1940s and 1950s, the term "album" began to be used to describe a collection of songs by a single artist, released together on a single disc. Today, the term still refers to a collection of songs, although it is more commonly used to describe a digital collection of songs or a physical collection released on CD or vinyl.