Columbia subject headings:
Information storage and retrieval systems--Directories.
Machine-readable bibliographic data--Directories.
Online bibliographic searching--Directories.
Harvard subject headings:
Bibliographical services--Directories.
Information storage and retrieval systems--Directories.
Machine-readable bibliographic data--Directories.
Online databases--Directories.
Princeton subject headings:
Information storage and retrieval systems--Directories.
Machine-readable bibliographic data--Directories.
Online bibliographic searching--Directories.