tubas (which are commonly made in pitches of BB, CC, EE, and F) the instrument's part is written in "concert pitch", not transposed by key for a specific instrument. Although sousaphones may have a more restricted range than their concert tuba counterpart (most sousaphones have 3 valves instead of 4 to reduce weight), generally they can all play the same music and usually have parts written in the bass clef and the indicated octave is played (unlike double bass or electric bass that sound an octave lower than the indicated note). Many older sousaphones were pitched in the key of E, but current production of sousaphones in that key is limited. Varieties Although most major instrument manufacturers have made, and many continue to make, sousaphones, Conn and King (H.N. White) instruments are generally agreed among players to be the standards against which other sousaphones are judged for tone quality and playability. Perhaps the most highly regarded sousaphone ever built is the Conn model 20K, introduced in the mid-1930s and still in production. Some players, especially those who find the 20K too heavy for marching, prefer the slightly smaller King model 1250, first made in the late 1920s and also still in production as model 2350. Historically, Holton, York and Martin sousaphones have also been considered fine horns. Very large bore (>= 0.750 inch) sousaphones, with oversized bells as large as in diameter, were made by Conn ("Grand Jumbo" [46K (3-valve) & 48K (4-valve)]) and King ("Jumbo" [1265 (3- & 4-valve versions)] & "Giant" [1270 (3-valve) & 1271 (4-valve)]) in the mid-1920s and 1930s, and by Martin, York, & Buescher, but they disappeared from the catalogs during the Depression or at the onset of World War II. Because of their weight and cost, few were made and even fewer survive, especially the 4-valve models. Fiberglass In recent years, sousaphones have been available made of fiberglass reinforced plastics instead of brass. The fiberglass versions are used mainly for marching, with brass instruments being used for all other situations. Fiberglass sousaphones can be found commonly in younger marching bands, such as middle schools, due to their lightened weight load. Depending on the model, the fiberglass version does not have as dark and rich a tone as the brass (King fiberglass sousaphones tended to have smooth fiberglass and a tone somewhat more like a brass sousaphone; Conn fiberglass sousaphones often had rough fiberglass exteriors