Here's why:
* Subject: The subject of a sentence or phrase is the person, place, thing, or idea that performs the action or is described.
* Complete Subject: The complete subject includes the noun (or pronoun) that is the subject and any modifiers (words or phrases that describe the subject).
In this case:
* Noun: "collection" is the noun acting as the subject.
* Modifiers: "a large", "of music manuscripts" are the modifiers describing the collection.
Therefore, the complete subject is the entire phrase: a large collection of music manuscripts.