In mainframe DOS (DOS/VS, DOS/VSE, etc.), the copybook

was stored as a "book" in a source library. A library was

comprised of "books", prefixed with a letter designating the

language, e.g., A.name for Assembler, C.name for Cobol, etc.,

because DOS didn't support multiple libraries, private

libraries, or anything. This term is commonly used by COBOL

programmers but is supported by most mainframe languages.

The IBM OS series did not use the term "copybook", instead

it referred to such files as "libraries" implemented as

"partitioned data sets" or PDS.