The double bass differs from the other members of the string family - the violin, the viola and the cello - in a number of important ways.
Firstly, it is physically different from the other strings rather than just being a scaled up version. The design of the double bass dates back to the days of the viol, an early design of string instrument in use from the 15th to 18th century, and to this day the bass has retained the viol's distinctive sloping shoulders. The so-called Dragonetti bow, held from below rather than above, is another feature more characteristic of the viol rather than the violin family.
Another unusual characteristic of the bass is that it is the only member of the string family that is a transposing instrument; in other words, the sound it makes is not the sound written in the part. The explanation is again largely historical. The traditional role of the double bass in the early classical orchestra was to echo or 'double' the part of the cellos, but playing it an octave lower in pitch to add greater sonority. As a result, the bass player was simply required to follow the music of the cellos and rely on his instrument to transpose it automatically into the correct pitch one octave lower. Although later composers started to write separate, unique parts for the bass, the practice of writing in the bass clef of the cellos has been retained to this day.
Yet another oddity of the double bass is that it is the only member of the string family to be tuned in fourths rather than fifths. The explanation here is physical: the much greater distance between the notes on the keyboard of a bass makes it impossible for a player to stretch further than an interval of a fourth, and the instrument is therefore tuned to the notes of low E, A, D and G. But unfortunately the matter does not end here. A lack of standardisation in the tuning of early basses has resulted in an occasional requirement (even in music as 'late' as that of Bach and Mozart) for lower notes down as far as C. This has resulted in a further curiosity of the double bass family: the five-stringed instrument, the fifth string being tuned specially to B or C to meet this awkward requirement.
In fairness, it should be said that the five-string bass is by no means a fully accepted concept, as many players insist that a five-stringer is acoustically inferior to the normal four-string instrument. Indeed, some players prefer a four-string instrument with a so-called 'piccolo extension', a rather ugly extension to the fingerboard which can be brought into play as required to extend the E down to B or C. Others players will turn a blind eye to the problem and discreetly play the offending notes an octave higher, or not play them at all depending on the whims of the conductor or composer.
The double bass is undoubtedly a member of the orchestra's supporting cast rather than an instrument with soloistic ambitions. However, Stravinsky composed a challenging duet for double bass and trombone in Pulcinella, and Mahler's famous Frère Jacques parody in his First Symphony gives the double bass a moment of real glory. In recent times, the instrument has received greater prominence thanks to the efforts of the Russian-born American composer and double bass virtuoso Serge Alexandrovitch Koussevitzky (1874-1951). Diana Wanklyn, principal double bass of the Hallé Orchestra, joined The Alderley Edge Orchestra for a performance of Koussevitsky's Double Bass Concerto in October 1997.