Although it's typical to tune orchestral strings using harmonics to match the pitches. For the cello, viola and violin, this is done by comparing the 2nd and 3r

Although it's typical to tune orchestral strings using harmonics to match the pitches. For the cello, viola and violin, this is done by comparing the 2nd and 3rd harmonics (at the octave and twelfth). For the bass, tuned in fourths, one compares the 3rd and 4th harmonics (at the 12th and second octave).