Could someone please explain what melodic fragments are? I’ve been told that if you see scale degrees 123 or 321 in the melody or bass it calls for an idiom. I’ve also heard that there are other melodic fragments.
Idioms or fragments in the language of music are called motives, ornaments or groups of notes, as the head motive, the mordent or the final group in the section of a sonata. Usually they are called phrases or a clauses.
The idioms your teacher mentioned will be developed by the neume and later as well by the technique of playing the organ, piano or any keyboard as there are notes of passages between thirds, fourths and fifths and change notes by the intervals of seconds.
look up the neume:
and especially the 3 notes neumes:
Such idioms are developed from the ornaments (melisma and ligatures) of the Gregorian chant (plain chant). You will find there a dozen of similar idioms.
"In Western music, the term "melisma" most commonly refers to Gregorian chant (the first definition of melisma by the Merriam-Webster Online Dictionary' is "a group of notes or tones sung on one syllable in plainsong"). However, the term melisma may be used to describe music of any genre, including baroque singing and later gospel."
But idioms are also built and developed by the technics of certain instruments: