What string instrument has the lowest pitched sound and why?

The string instrument with the lowest pitched sound is the double bass. It is designed to produce deep, resonant tones due to its large size and longer strings, which vibrate at lower frequencies compared to smaller instruments. The double bass typically plays in the bass range in orchestras and ensembles, providing a foundational harmonic support. Its construction and tuning allow it to reach pitches that are significantly lower than those of other string instruments.