Actual play scripts are considered literature. They are classified by author along with the author's poetry, prose fiction, and correspondence. A single play would not be listed under a subject heading such as "England--Kings and rulers--Drama." A collection of plays about the kings and queens of England might be retrieved by such a search, but most likely you would miss a lot of relevant titles.
A known-item search is the most efficient. If the author and/or title of a play is known, a simple author, title, or keyword search will return relevant material. You might have to use a source outside the catalog to identify a play of the type that is desired, e.g. one-act plays, plays set in Kansas, etc. The set of reference books entitled Play Index does exactly that. Armed with an author and title -- i.e. an author and title -- an efficient search of the catalog is far easier.
The Library owns or has access to a wide variety of art information sources -- some are printed, others are available online. They are listed on separate pages below.
Keyword searching is the default mode but subject searching (under Advanced Search) generally gets more focused results.
For complex topics, select advanced search, located below the search box.
Here are the broad Library of Congress classification numbers for theatre materials: