It was not until the 20th century that a fourth string became normal for double basses in a symphony orchestra. This was the additional E string, a fourth below the standard A¢ string. In the 19th century, despite every other instrument becoming more chromatic and being required to be able to play every note a composer demanded, the double bass remained unaffected and pitches below A were transposed up an octave. This, of course, meant that the melodic line being played at double bass pitch (an octave below the cello) was for a moment lost.
In most Haydn and Mozart symphonies the double bass merely doubles the cello, but in Beethoven and later it acquired an independent line. The scherzo of Beethoven’s Fifth Symphony is a good example of the double bass’s new, 19th-century independence.