We call the interval between the note played by the 1/3 length and the note played by 1/2 the length of the same string a perfect fifth (if you sing the children's song 'Baa Baa Black Sheep' t...We call the interval between the note played by the 1/3 length and the note played by 1/2 the length of the same string a perfect fifth (if you sing the children's song 'Baa Baa Black Sheep' the first Baa and Black are a fifth apart).