* The libraries' hours are posted on their websites. (This is a simple and straightforward example.)
* The libraries' combined collection boasts over a million books. (This highlights the collective nature of the possessive.)
* I visited several libraries' websites to find the book I was looking for. (This shows the plural possessive being used with a descriptive phrase.)
* The libraries' recent renovations have made them much more inviting. (This demonstrates the plural possessive being used with a descriptive clause.)
Remember, the plural possessive of "libraries" is formed by adding an apostrophe and an "s" ("libraries'").