There are more violins in an orchestra than any other instrument, and a first-chair violinist is the orchestra leader. There are first, second and third violin sections, all playing different arrangements. The first violins typically play the melody, while the seconds and thirds back them up. The first violinists sit two to a desk on the conductor's left, with the leader at the front. The seconds sit on the row inside from the firsts, and the thirds on the next row.
The viola is slightly larger and is pitched slightly lower than the violin, with the strings running C, G, D, A, with the lower C tuned an octave below middle C. In an orchestra, the violas supply a simple backing to the complex violin arrangements. Violas tend to play the same parts as the third violins. The viola players sit in the center of the string section, between the violins and the cellos.
Cellos are much larger than violins. The instrument is balanced on the ground and held between the players' legs while they bow the strings horizontally. The cello is pitched one octave down from the viola. Cello music is arranged to provide a bass part to the violins. However, the cello is also capable of reaching high pitches, which means they can also carry a melody. The size of the cello section roughly corresponds to that of the first violins.
The double bass is the largest string instrument, and like the cello it rests on the ground between the player's legs. They are the lowest-pitched instruments in the string section, tuned E, A, D, G, with the highest G string corresponding to the G on the cello. The double bass provides the bass part for the orchestra's strings. A typical orchestra will have six to eight of them. They sit behind the cellos at the back right of the string section.