From Music Theory for the 21st-Century Classroom (Hutchinson, 2019):

“Musical form is full of sections, and the phrase is the smallest category of section. Unlike a motive, a phrase gives the sense of completing a formal unit.”

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Most melodic ideas, or “themes”, are structured as musical phrases.

Though usually thought of melodic, phrases incorporate melody, harmony, & rhythmic features.

• Made up of motivic ideas, smaller cells of musical material
• Tend to be evenly-organized (4, 8, 16 measures): Most commonly 4 or 8 measures
• Have a beginning, middle, and end.
• In common-practice styles, the understanding of a musical phrase is dependent on the harmonic progression and its cadential goal.

Some example musical phrases:

Beethoven – Piano Sonata No. 1, Op. 2, No. 1, Mvmt. I (1795)
(Recording on youtube)

Mariah Carey: “All I want for Christmas is You” – Chorus (1994)
(Recording on youtube)
There are *non-diatonic* chords in this example!