"The Rule of Three" - I love the name, it sounds like a Sherlock Holmes story! Nothing so exciting, of course: it is the name commonly given to a cataloguing rule from the Anglo-American Cataloging Rules, and it's been there since the 1970s. (Wikipedia seems to think differently. "Inherently funnier, more satisfying"? hmmm). As AACR2 is on its way out, so too is this cast-iron rule, and it can't be allowed to pass without comment.
Here it is :
If responsibility is shared among more than three persons or corporate bodies and principal responsibility is not attributed to any one, two, or three, enter under title. Make an added entry under the heading for the first person or corporate body named.
[from AACR2, 1988, but the rule predates this edition].
RDA, the new code which is due to replace AACR2, takes quite a different view, abolishing the idea of a "main entry", allowing all the authors to appear in their own right in the catalogue, and leaving far more policies up to "cataloguer's judgment".
What was the reason for the Rule of Three? It always seemed a bit mean to cut the authors off arbitrarily. A book with one, two, or three authors would include an entry in the catalogue for all three authors: more than that, and they were abruptly relegated to an added entry for the first author, and not even a whisper of a mention for the others. The trouble with it, at least for academic libraries, was trying to stick to the rule in the face of upset authors (often members of the academic staff known to the librarians) who didn't take kindly to being airbrushed from the catalogue.
Cataloguing rules seem to be a mystery to non-cataloguing librarians, who probably think that the cataloguers in their institutions make all these things up as they go along. We have had librarians of long standing (caught between intransigent cataloguers and miffed authors) insisting to us that they have never heard of this rule. Making things up might have been possible when each catalogue was a separate being in its own institution, but these days everything is visible on the web and many records are shared. This saves a lot of time and money and avoids the waste of duplication. It also means you need the rules even more - all that saving of time and avoiding of duplication goes down the drain if you are then constantly altering policies to suit local needs or tinkering with individual records on a random basis (i.e. when someone complains).
Like many rules, it dated from the days of the card catalogue, and it was probably partly designed to reduce the typing and filing of catalogue cards. Three was as good a cut-off point as any. Technology moved on long ago, and the idea of the "main entry" is no longer so important in the catalogue (it's still useful for allocating suffixes to classmarks for shelving purposes). While there is some additional work involved in including all the authors of a multi-author work, it is not the laborious drawer-filling process it would have been in terms of extra cards. There was a bit more to it than that, though: AACR2's definition of a personal author is "the person chiefly responsible for the creation of the intellectual or artistic content of a work" (21.1A1), and once you get beyond three authors you might well wonder whether you can say this of all of them. The "main entry" concept is also still as relevant as ever in bibliographic citation, which can these days be derived directly from the catalogue.
I think I can safely say that of all cataloguing rules the Rule of Three has probably been the cause of most complaints over the years, at least in the academic libraries I have worked in. Heddwch i'w llwch, as we say in Wales.