Solmization, the practice of assigning syllables to the different “steps” of the musical scale, originated in ancient India. Fast forward a few thousand years to 6th-century Spain, when Isidore, the Archbishop of Seville, lamented that “unless sounds are remembered, they perish, for they cannot be written down.” A Benedictine monk named Guido d’Arezzo, who was also a master of music, then began working on a system to prevent sacred tunes from being lost to history.

D’Arezzo was familiar with solmization, and noted that most of the Gregorian chants popular at that time could easily be learned by singers if they could see the tone progression up and down the scale and associate it with the sound. He assigned the notes of the scale—C, D, E, F, G, A, B, C—a syllable: Do, Re, Mi, Fa, Sol, La, Ti, Do. Yes, it actually is sol: it’s traditionally written that way when the tonic notes are spelled out, and often referred to colloquially as the Solfa or Solfège scale. That final L is hard to hear because of the la that follows.

Those weren’t just random sounds he chose. They came from “Ut Queant Laxis,” a well-known hymn in the Middle Ages that was chanted for vespers. Each succeeding line of the song started one note higher than the previous one, so Guido used the first letters of each word of each line: Ut queant laxis, Resonare fibris: Mire gestorum , Famuli tuorum: Solve, and so on. Ut was eventually deemed too difficult pronounce and was changed to do.

Did Guido d’Arezzo’s method work? Well, as Julie Andrews (via Rodgers and Hammerstein) put it in The Sound of Music, “When you know the notes to sing, you can sing most anything!”

A version of this story was published in 2013; it has been updated for 2023.

This article was originally published on www.mentalfloss.com as Why Are the First Notes of a Tonal Scale Called ‘Do, Re, Mi’?.