MARC DTD: Background and Development

The term "MARC DTD" (MAchine Readable Cataloging Document Type Definition), refers to implementations of Standard Generalized Markup Language (SGML). SGML is a technique for representing documents in machine-readable form which was approved as an international standard, ISO 8879 (Information processing--Text and office systems--Standard Generalized Markup Language). It was developed to fill the need for a non-proprietary standard for text encoding so that machine-readable data could be exchanged between dissimilar text encoding environments. SGML is widely used in the publishing industry where documents are created using various computer systems.

http://www.loc.gov/marc/marcdtd/marcdtdback.html