What is the term used when high instruments play the same notes as the soprano line and lower instruments play the same notes as lower voices?

The term used when high instruments play the same notes as the soprano line and lower instruments play the same notes as lower voices is "doubling." This concept refers to the practice of having different instruments play the same pitches as vocal lines, effectively reinforcing those melodies. For example, if a soprano is singing a particular note, and a flute or violin plays that same note, the flute or violin is said to be doubling the soprano. This technique can create a fuller sound and emphasize melodic lines in a composition.

Harmonizing refers to when different musical lines combine to create chords or harmony, which is not the case here. Contrapuntal deals with the idea of independent melodic lines interacting with one another, rather than just duplicating lines. Layering typically refers to the addition of multiple musical elements to build texture, but it does not specifically address the concept of duplicating pitches across different instruments. Doubling serves an important role in orchestral and vocal music, enhancing the overall texture and emphasizing key melodic ideas.