Since World War II the contrabass trombone in F with two valve attachments has been primarily in use in orchestras, though the 18′ B version is still used by many. Originally due to reasons of limited space conditions in opera orchestra pits, the bell section was provided with a coil to reduce the length of the bell bow, but since the 1970s the long, straight form has taken precedence. Through the combination of both valves, the extension handle on the outer slide also became redundant and the instrument is provided with five or six working positions on the slide. Instruments today are typically built in two configurations: the traditional style with two valves lowering the pitches to E♭ and B♭ (combined placing the instrument in A♭) and the “American” style with two valves in C and D♭ (combined to place the horn in A.) Some instrument makers provide special tuning slides that allow for changing the instrument to either configuration. Technical passages on the horn are generally able to be played with more agility than the double-slide contrabass trombone in B♭, since for much of its range it requires a shorter column of air to vibrate and has two valves instead of one, enabling more alternate positions. Nonetheless, the instrument is best suited to more harmonic material, not unlike a tuba, rather than virtuosic melodies.