Examines some of the bibliographic advantages of a union catalogue with a central database over a distributed, or "virtual" union catalogue. Discusses the nature of these bibliographic advantages in the context of interlibrary document delivery, and also the circumstances which produce them, and make them more significant. Reports on a brief study of the extent to which two major library catalogues in Singapore have diverged following the adoption of a distributed model. This indicated that the bibliographic content of a distributed union catalogue may be significantly poorer than that of a central database, and in particular in terms of more: duplication; inconsistency; errors; and omissions. There are at least four important reasons why this may be so.