MONOCHORD. — The monochord has been for music the instrument with the most varied applications: first an experimental tool for acoustics, it became, from the 9th century onward, a didactic instrument; finally, after the 14th century, under the name of trumpet marine (Trum-scheit), it became a bowed instrument used in churches during the 18th century, especially by nuns, whence the name Nonnengeige it bore in Germanic countries.
The monochord essentially consists of a string stretched over a soundbox, of variable length, by means of a bridge. Oral traditions, recorded in the writings of Odo, Ubaldo, Guido, Adelbold, Berno, Notker Labeo, William of Hirsau, Walter Odington, Johannes de Muris, etc., determined the quality, length, and thickness of the string and soundbox, as well as the various procedures for obtaining divisions of the string that would produce the various intervals, generally tones, possible within the compass of an octave and a fifth or two octaves.
The invention of the monochord is attributed to Pythagoras, who is said to have learned of it from the Egyptians. The first treatise to mention it is that of Euclid (c. 300 B.C.) under the title *Sectio canonis*. Boethius (*De institutione Musicae*, IV, chaps. 6–12) derives the chromatic and enharmonic intervals from the divisions of the monochord. While Boethius and Ubaldo use Greek and Latin letters to mark the successive sounds, after Odo it became customary to mark on the soundbox itself, with the first seven Latin letters, the various points where the bridge should be placed before setting the string in vibration with the finger or a plectrum.
The use of the monochord as a pedagogical instrument developed with the progress of musical notation, first imprecise in many neumes (v. NOTATION), then written on a staff and decipherable without further reliance on the teacher’s memory. The multiplication of attempts at alphabetic notation at the end of the 10th and in the 11th centuries is to be linked to the greater ease for the pupil of finding on the monochord the alphabetic letter read earlier on the parchment and thus the desired pitch. A treatise, *Musica theoretica*, falsely attributed to the Venerable Bede (PL 110, 911–20) and recognized as the work of a certain Master Lambert (cf. K. Schlesinger, *The Greek Foundations of the Theory of Music*, in *The Musical Standard*, 27 [1926], pp. 198 ff.), shows how in the 13th century the accuracy of intonations in the polyphony of the *Ars nova* was checked on the monochord, since this author explains how to obtain simultaneous consonances on II.