To fill this gap in the literature, mandolin orchestras have traditionally played many arrangements of music written for regular orchestras or other ensembles. Some players have sought out contemporary composers to solicit new works. Traditional mandolin orchestras remain especially popular in Japan and Germany, but also exist throughout the United States, Europe and the rest of the world. They perform works composed for mandolin family instruments, or re-orchestrations of traditional pieces. The structure of a contemporary traditional mandolin orchestra consists of: first and second mandolins, mandolas (either octave mandolas, tuned an octave below the mandolin, or tenor mandolas, tuned like the viola), mandocellos (tuned like the cello), and bass instruments (conventional string bass or, rarely, mandobasses). Smaller ensembles, such as quartets composed of two mandolins, mandola, and mandocello, may also be found.