Monochord is a one-stringed instrument with movable bridges, used for measuring intervals. The first monochord is attributed to Pythagoras.
The story is told that Pythagoras wished to invent an instrument to help the ear measure sounds the same way as a ruler or compass helps the eye to measure space or a scale to measure weights. As he was thinking these thoughts, he passed by a blacksmith’s shop. By a happy chance, he heard the iron hammers striking the anvil. The sounds he heard were all consonant to each other, in all combinations but one. He heard three concords, the diaspason (octave), the diapente (fifth), and the diatessaron (fourth). But between the diatessaron (fourth) and the diapente (fifth), he found a discord (second). This interval he found useful to make up the diapason (octave). Believing this happy discovery came to him from God, he hastened into the shop and, by experimenting a bit, found that the difference in sounds were determined by the weight of the hammers and not the force of the blows. He then took the weight of the hammers and went straight home. When he arrived home, he tied strings from the beams of his room. After that, he proceeded to hang weights from the strings equal to the weights he found in the smithy’s shop. Setting the strings into vibration, he discovered the intervals of the octave, fifth and fourth. He then transferred that idea into an instrument with pegs, a string and bridges. The monochord was the very instrument he had dreamed of inventing.