Nothing is more practical than a good theory. A banal statement, considering that a theory should always enable its users to easily derive the statements they need for practice.
But a theory for catalogs or cataloging? Is that really necessary? A question anyone is likely to ask who has never been confronted with the matter nor considered it with any seriousness.

Using Internet search engines, and knowing their operation is fully automated, people tend to view with skepticism all practical and theoretical effort invested in catalogs. Any good search engine, however, has to be be based on a good theory - though that one may differ quite a bit from a catalog theory.
 

What do libraries and the Internet have in common?

Both provide access to collections of recordings. One need not use the difficult-to-define concepts of information and knowledge here. We may leave it open whether or not an "information society" exists, or a "knowledge economy", and whether everything is information or knowledge that is squeezed between book covers or on Web pages. The "Pisa Studies" have reminded everybody that knowledge doesn't come without learning. Being in the possession of printed matter does not mean to possess knowledge, but printed text turns into living knowledge only after reading and understanding, and then this knowledge will sit in the head of the reader and not on the paper or screen. Nobody will doubt that ours is a learning society, and recordings of experience and insight are of central importance for learning. One learns from direct interaction between humans, by one's own doing, by observation, or through studying. Which mostly consists of taking in what others have recorded.

In many cases, suitable recordings have to be found first. Millions of humans, over millennia, have recorded their experience and encounters, their findings, their insights, and their inspiration. When this started with the Greeks, Platon saw in it a symptom of decline: people wouldn't exercise their memories any more because they would now rely on inferior surrogates. But people did not stop at making use of their own recordings, they started using those of others as well. Collecting began. Libraries were created. After collecting more than a few hundred papers or papyri, a system of ordered shelving had to be invented or else the usefulness of the collection would have suffered.

How did cataloging come about?

Once several thousand items have been collected, their physical arrangement, whatever the system, becomes tedious. One will need finding aids, i.e., secondary recordings (or meta-recordings), which will reveal where in the collection a particular item is located. This is the birth of cataloging: it shifts the process of ordering from the shelf to paper, to files or, nowadays, to databases. Unless one also invents a nice theory along with this, the usefulness of the catalog will diminish with its size rather than increase.

Once one has millions, the assembling of the finding-aids in itself becomes quite a considerable effort. No wonder there are attempts at automating the process, at least for collections that exist in digital formats. The metaphoric term "search engine" suggests, misleadingly, that a machine peruses the documents as such, focusing on their content. The actual searching is, however, always performed on surrogate files the system constructs for this purpose. Software can only match character strings, not concepts or ideas. This cannot be done in just some arbitrary way, but there has to be a systematic way, an algorithm, which means a theory.
 

Contents of libraries and Internet

Combine libraries, archives, and the Internet, and they comprise nothing less but the accumulated intellectual and artistic recordings of humankind, inasmuch as these survive, from all periods, all countries and cultures, in all languages and scripts and about all subjects, by all individuals who ever wished to make a contribution. The size and the complexity of this is staggering. It is naive to expect that navigating this multidimensional universe might be easy or might be made a simple matter. One may try to simplify the description of the world, but the world itself will not become any simpler that way. Note that the initial enthusiasm of the metadata movements has softened a bit...
A catalog attempts to help with finding documents and with orientation among documents, and internet search engines strive to do the same. The question is: in what way, with what principles and methods, by what theory can or should they work in order to help the most people in the largest number of cases in the best possible way. No one single method can serve all purposes and all searchers all the time - everybody will know this who has tried to find anything on more than one occasion.

Books or Internet - a matter of taste?

There is not really an either-or situation. Only the combined contents of both worlds constitute the complete universe of recorded knowledge and achievement. Library catalogs on the Internet do not change this, be they as comfortable as they may, because catalogs carry only descriptions, not the publications themselves which exist only on paper or in microform. To digitize all of these and make them full-text searchable is presently utopia: there are many millions of texts and new ones continue to be produced by the tens of thousands per year, a great many of which are not in machine-readable formats, Google's efforts notwithstanding. Catalogs contain only very brief and standardized descriptions of the documents, whereas for internet content full text is the norm. But: diversity is enormous, and most documents lack a standardized description (a.k.a. "metadata"). From this it follows there will be a number of differences between catalogs and internet search engines. In libraries, we not only have to understand that but we should also be able to transfer this knowledge to our readers.
Further down we make an attempt to juxtapose catalogs and search engines in a table.

First, however, let us look at catalogs as such, and at the difference between the contemporary device, the OPAC, and the card catalog (now gathering dust, if not discarded). We also have to ask what consequences should be envisioned for cataloging rules. It goes without saying that the OPAC is here to stay and that card catalogs are history, but one may still learn from a comparison.
For readers who want more detail, there's the introductory chapter of an outstanding book: Martha M Yee's and Sara Shatford Layne's "Improving Online Public Access Catalogs" (ALA, 1998. - ISBN 0-8389-0730-X).

What is the principal problem today in searching?
The true problem with OPACs is no longer, as it was for cards, that users have difficulty just finding something at all. Instead, for most queries the OPAC does bring up some results - then, however, there is no easy way of knowing if this is all there is and whether the best-suited items have at all been brought up. Users, in other words, cannot know they have missed something or even a lot or even very important things. Their awareness of this is generally low, and catalog use studies have shown that it is very difficult to entice users into making several different attempts - or briefly, to set them thinking. Their confidence in technology, in other words, is improperly high. What most of them use is just the standard or default options, making barely more than one attempt. This is probably based on an overall least-effort tendency, or on the unreflected assumption that what's offered as default is also the best possible way and others are inferior. The catalog itself cannot overcome this. The catalog may be as good as it gets, that's not the point. Users have to think and judge for themselves, today as much as yesterday, and this is not going to change with any new generation of technology. And they should even be happy about this, for otherwise they themselves might be replaced by machines... Be that as it may:  there certainly have to be easy ways of searching for simple questions, but ambitious and knowledgable users should also be provided with and invited to use sophisticated techniques.

What is a good catalog?

• Reliability: Starting from a citation, one should be able to ascertain quickly and with certainty if the item is in the collection or not. (In an unreliable catalog, esp. the latter may require many trials before one can be sure. Catalogs need to have that feature because of acquisitions checking, for example, or to find out whether an ILL order is necessary.)

• Serendipity :  Browsing functions are essential, firstly because one doesn't always have precise search criteria, and secondly because chance findings are sometimes valuable. That's one reason why users tend to go to the stacks first when they know the arrangement. Catalogs should therefore make related materials browsable - the question of course being, what exactly is "related"?  OPACs can, for example, support browsing in these ways
1. provide alphabetical indexes of names, terms, titles etc., browsable up and down,
2. present result sets in more than one arrangement for the user to choose, and
3. make related publications accessible via hyperlinks (for subject terms, classification codes, names).

• Depth :  This covers two aspects that are not exactly part of cataloging:

1. a policy saying  what materials or objects are subject to cataloging. Classically, these are books, meaning self-contained knowledge packages. More often than not, however, a book consists of several or even many packages of recorded knowledge, each of which representing a unit that might become the subject of a bibliographic record itself - because someone might well be searching for it. Just think of proceedings volumes or festschriften, not to mention periodicals. With the exception of belles-lettres, readers will, in many cases, be interested and thus actually be looking for a chapter or parts of a book rather than the whole volume. If cataloging restricts itself to title page information, the catalog will be completely oblivious to all the constituent parts of books. For economic reasons (labor, space), not many libraries have ever done chapter-level cataloging. One important case are "multipart publications" with individually titled volumes: are these to be cataloged as a whole or each volume separately - or both? The focus of European cataloging seems to have been heavily on the parts, whereas American catalogers have more often only perceived the whole.
2. a concept for subject indexing. Is it enough to assign a few subject terms and/or classification symbols to a document to nail down its content matter, or should the aim be to index every subject that is actually dealt with in some part of the publication?  There are experiments, for example, with tables of contents of books (as in OhioLink). There are also experiments with automatic assignment of additional terms or notations by software.

From one dimension to many

The most decisive difference between conventional and online catalogs is this:
(We are not talking about technical differences here, like availability around the clock around the globe, just catalog theory!)

Card Catalog:  a linear sequence of entries, i.e., a one-dimensional space, the ordering principle being the alphabet on the lowest level, names/titles/subjects on an upper level. Some libraries had several catalogs for two or more time periods or for otherwise defined parts of their holdings. Every document can be represented by more than one card in several places of the sequence, one of these being called the "main entry". It served two purposes. Firstly, that of collocating related works in one place (like an author's works under an established form of his/her name). Its second and probably more important function was to provide a predictable location for the item in the catalog: if one knew the principle, one was able to find with certainty what one was looking for in just one attempt. Practicability limited the number of cards per item to an average of well below ten. There are many conceivable ways of arranging a card catalog, and in particular, of determining the entries to be represented in it. The pattern, once chosen, has to be followed consistently in order for the catalog to be reliable. Therefore, a card catalog is the utmost extreme of pre-coordination. Very elaborate rules had proved to be necessary in order to establish the pre-coordination.

OPAC:  in principle, it contains an unordered mass of structured records. Software, however, can easily produce a dozen or more different indexes, each being a linear, sorted sequence of certain parts of the records. Logically, these are still quite like card sequences, but then software, processing a user query, can extract arbitrary subsequences and merge or intersect them with subsequences from one or more of the other indexes, yielding subsets of the database which can then be presented in one or more different meaningful arrangements. Criteria like names, titles, numbers, subject terms etc. may thus be combined in all conceivable ways. Indexes are thus like the axes of a multi-dimensional space in which software enables the user to navigate. Multi-dimensonal spaces are abstract, mathematical entities and therefore present a challenge for many users to comprehend. As opposed to card catalogs, it means that OPACs rely heavily on post-coordination.

The actual arrangement of the pre-coordinated card sequence results from two decisions:

1. Entries: What are the criteria for the selection of entries  - which persons or other entities are to be represented by a card for a main or added entry, and which not?
2. Headings: What is the exact spelling of the card headings for the selected entities?
   The difficulties encountered here gave rise to the whole edifice known today as authority control.

Standard situations of catalog use

The situation most frequently encountered is probably the factual search: For this, catalogs are not very helpful because they contain descriptions of reference works only, not their contents. Search engines, however, index the available documents directly and in their entirety and can thus lead immediately to the facts contained therein. When looking for facts, search engines are therefore the first stop for most anybody these days: the engines serve as directory, dictionary, encyclopedia, atlas, calendar, timetable, picture book, etc. Catalogs can only point users to all those reference tools , which makes the search for facts more cumbersome and time-consuming.
If, however, we turn to document searching, we can observe at least three broad categories of situations frequently encountered when people use a catalog or search engine:

(a) Known item search ("I know exactly what I need"): looking for something cited or referred to in some other place, like a bibliography (before the advent of hyperlinks).

• Some users don't know there is a newer (better, more complete) edition than the one he/she has been referred to.

• A citation may be imprecise but still good enough to find at least one edition - this one should then lead to the others.

• Users are sometimes happy with any edition of a cited work, no matter the real title.

• Users may enjoy the serendipity resulting from being presented with more than one edition.

Perfection, however, is out of reach: for example, very often a library has only one edition of a work and the cataloger is unaware of the existence of others (esp. ahead of time before other editions would be published!). Then, only this edition can be found in the catalog, but not under any other title by which it may be known to a searcher. Such cases are less frequent in large, shared databases.

Plus ça change, plus n'est pas la même chose ...

Technology enabling proliferation like never before, it is now very common to encounter diverse "manifestations" of a text: the same content can be presented in different versions or file formats and with all sorts of modifications. This can aggravate the difficulties with collocation searches (situation (b)). And titles, though being the most important element identifying a document or work, are not handled with a lot of care in the Internet.
Classically, the manifestation problem varies from one discipline to another. It is probably least virulent in the sciences and in technical disciplines, for it is rather an exception for a document to live through more than one edition. In belles lettres, it is more common, but music has arguably the most and the worst examples: many pieces can be found in dozens of interpretations, titles changing all the time as well as the forms of names (Tchaikovsky!). The "uniform title" is therefore nowhere as important as in music to bring all editions or versions together.
 

AACR are concerned with the formal level, not the subject level!

The AACR code of cataloging rules, like the German RAK, deals with situations (a) and (b) only. These pose problems that can be solved by purely formal or descriptive means,  whereas (c) requires attention to the content of things cataloged.
In the world of cards, there were sometimes (in Germany, nearly always) separate catalogs for situation (c). OPACs, however, always combine formal and subject access points in the same database if not generally in the same index. They can differ in having or not having an "anyword index" actually combining all words (but not phrases) occurring in bibliographic records. In any case, it seems important to have uniform access forms for personal and corporate names, serving for both kinds of accesses. German rules are not yet fully unified in this regard.

The problems described here have been known at least since Antonio Panizzi's work at the British Museum in the 19th century (his "Ninety-One Rules" were published in 1841). He had set himself the task of setting up the first complete catalog for the library. His employers found his ideas somewhat overly complicated and were reluctant to support him. This situation keeps repeating itself...

Attempts at formulating international principles for cataloging set in only in the mid 20th century, the all-time highlight being the IFLA Conference of 1961 in Paris. The "Statement of Principles" promulgated there became the foundation for AACR as well as for RAK. Only as late as 1999,  IFLA came up with a new milestone paper, entitled "Functional Requirements of Bibliographic Records (FRBR)", which is gaining ground not just in library circles but also in metadata projects. Some of its main points are presented in a separate paper, "What should catalogs do?".  New cataloging rules are in the making under the name of Resource Description and Access (RDA)., to supersede AACR in 2009. FRBR is aimed at collocation search more than anything else, trying to say with all due precision in what ways entities can "belong together" and in what ways that fact should be reflected in catalogs.

Is AACR2 inextricably intertwined with MARC21 (and RAK with MAB)?

Catalogs and search engines

Time and again, catalogs and search engines are juxtaposed in a pears vs. apples comparison.

The intention here is not to find out which is the better gadget but to show what differences exist. Not just librarians may be interested to get a clearer picture of strengths and weaknesses.

There is actually no competition, for catalogs and search engines cover different ground and cater to different needs. Most print material remains offline and thus inaccessible for harvesters, and on the other hand, many online resources have unprintable characteristics and thus could not be published in print.