An album cover (also referred to as album art) is the front packaging art of a commercially released studio album or other audio recordings. The term can refer to either the printed paperboard covers typically used to package sets of and 78-rpm records, single and sets of LPs, sets of 45 rpm records (either in several connected sleeves or a box), or the front-facing panel of a cassette J-card or CD package, and, increasingly, the primary image accompanying a digital download of the album, or of its individual tracks. In the case of all types of tangible records, it also serves as part of the protective sleeve. Early history Around 1910, 78-rpm records replaced the phonograph cylinder as the medium for recorded sound. The 78-rpm records were issued in both 10- and 12-inch diameter sizes and were usually sold separately, in brown paper or cardboard sleeves that were sometimes plain and sometimes printed to show the producer or the retailer's name. These were invariably made out of acid paper, limiting conservability. Generally the sleeves had a circular cutout allowing the record label to be seen. Records could be laid on a shelf horizontally or stood upright on an edge, but because of their fragility, many broke in storage. German record company Odeon pioneered the "album" in 1909 when it released the Nutcracker Suite by Tchaikovsky on four double-sided discs in a specially designed package. (It is not indicated what the specially designed package was.) The practice of issuing albums does not seem to have been taken up by other record companies for many years. Beginning in the 1920s, bound collections of empty sleeves with a plain paperboard or leather cover were sold as "record albums" (similar to a photograph album) that customers could use to store their records. (The name "record album" was printed on some covers.) These empty albums were sold in both 10- and 12-inch sizes. The covers of these bound books were wider and taller than the records inside, allowing the record album to be placed on a shelf upright, like a book, and suspending the fragile records above the shelf, protecting them. Starting in the 1930s, record companies began issuing collections of 78-rpm records by one performer or of one type of music in specially assembled collections. These albums of several 78-rpm records could include a collection of popular songs related by either performer or style,