The noun libraries' is the plural possessive form (the possessive form of the plural noun). The apostrophe at the end of the noun indicates that something belongs to two or more libraries.
Examples:
This city is know for its many fine libraries. (plural)
All of the libraries' hours are posted on-line. (plural possessive)
Buses' is plural possesive.
Singular: hypothesis Plural: hypotheses
dictionaries'
The plural possessive of girl is girls'.
The plural possessive form of men is men's.
Plural possesive.
The Peoples' court :}
Libraries is the plural form of library.
The plural of library is libraries. As in "libraries have lots of books".
The possessive form for the irregular plural noun libraries is libraries'.
For plural nouns that end in -s add an apostrophe. - parents' The parents' cars were in the driveway.
The plural is "libraries".