Fixing Priority in Classification: General Rules and Best Practices

Every classifier eventually meets the same problem. You analyse a document, reach for the schedules, and find that two or even three class numbers seem to fit. A book might sit comfortably under a broad subject and under a narrower one. It might belong to a material on one hand and to a process on the other. So which number wins? Fixing the priority of numbers is the skill of settling these ties in a consistent, defensible way. Beyond the well-known step-by-step procedures, classification systems offer a set of general rules that quietly guide these decisions. This post unpacks those rules so you can apply them with confidence.

What “fixing priority” actually means 🔗

In any large scheme such as the Dewey Decimal Classification (DDC) or the Universal Decimal Classification (UDC), a single subject can surface in several disciplines and in several places within one discipline. The DDC itself is built around ten main classes that each split into ten divisions and then ten sections, so the same idea can legitimately appear at more than one point in the notation. When this happens, the classifier must choose one number to represent the document on the shelf. Fixing priority is simply the disciplined process of making that choice.

The goal is never to find a “correct” number in some absolute sense. The goal is consistency. If two librarians classifying the same book arrive at the same number by following the same rules, the system works. The rules below give everyone a shared logic to follow.

Start with specificity: choose the most relevant number 🔗

The first and most important guideline is specificity. When more than one number could apply, choose the one that pins down the subject as precisely as possible. This is often called close classification, where the notation specifies the content of a work to the fullest reasonable extent.

Consider a book on organic chemistry. You could file it under the broad science number for chemistry, or under the more precise number reserved for organic chemistry. The specific number is the better choice because it tells the reader exactly what the book is about and groups it with its true neighbours. A reader hunting for organic chemistry should not have to wade through general works on all of chemistry to find it.

Why specificity helps the reader 🔗

Classification exists to bring like documents together and to separate unlike ones. A broad number scoops up too much. A precise number creates a tight, browsable shelf where related titles sit side by side. The structure of these schemes is hierarchical, so longer notation generally signals a narrower, more specific subject. The principle is straightforward: never settle for a general number when a specific one accurately describes the document.

The rule of zeros: fewer zeros usually win 🔗

A subtler pattern appears once you start comparing notation digit by digit. Many schemes treat the digit zero as a kind of filler used to pad out a number or to introduce a standard subdivision. When you have a genuine choice at the same point in the hierarchy, the convention is to avoid the option that begins with a zero.

This is the rule of zero. It instructs that subdivisions beginning with zero should be avoided when there is a choice between a zero and a digit from one to nine at the same place in the notation. The same logic extends to numbers beginning with double zeros, which should be passed over in favour of the digit that carries more subject meaning.

Why fewer zeros signal more meaning 🔗

In the DDC, the system uses the convention that no number should have fewer than three digits, so zeros are added to fill out the notation. A zero in these positions often does not represent a real subdivision of the subject; it simply holds a place. A non-zero digit, by contrast, usually carries actual subject content. So a number with fewer zeros tends to point at a more defined topic, while a number heavy with zeros points at something broad or merely formal. When two numbers compete, the one expressing more genuine subject meaning is the safer pick.

Preferring the concrete over the abstract 🔗

A third principle helps when a document straddles a tangible thing and an abstract idea about it. As a general rule, give priority to the concrete subject over the abstract one. A concrete subject is a real object or a thing being acted upon, while an abstract subject is a concept, theory, or property considered on its own. The distinction between a concrete and an abstract treatment is old and well established, and it maps neatly onto classification choices.

This idea is closely tied to what the DDC calls the rule of application, which directs you to class a work dealing with interrelated subjects with the subject that is being acted upon. For example, an analytical work on Shakespeare’s influence on Keats is classed with Keats, because Keats is the subject receiving the influence. The thing being acted upon is treated as the real focus of the work. Importantly, this rule takes precedence over the other rules when it applies, so it is worth checking for first.

When subjects are treated equally: the first-of-two rule 🔗

Sometimes a work covers two subjects with no clear sense of one acting on the other, and both receive equal treatment. Here the first-of-two rule steps in. It instructs you to class the work in the number that comes first in the schedules, provided the two subjects are not used to introduce or explain one another and no other rule overrides the choice.

A classic illustration involves a history that deals equally with two countries. A work on the history of the United States and Japan, where the United States is even named first in the title, is classed with the history of Japan, because Japan’s number comes earlier in the schedules than the number for the United States. The rule looks only at the order of the numbers, not the order of words in the title. This rule is set aside whenever a specific instruction in the schedules, the rule of application, or a comprehensive number tells you otherwise.

Worked examples: putting the rules to work 🔗

The principles become clearer when you trace them through real decisions. The three short cases below show how a classifier weighs the options.

Social aspects of relief 🔗

Suppose you are classifying a book on the social aspects of relief, meaning organised welfare and assistance to people in need. The phrase “social aspects” might tempt you toward a general sociology number, but that would be too abstract and too broad. The concrete subject here is relief and welfare itself, which lives within the social problems and services area of the social sciences. By preferring the concrete subject acted upon over the abstract framing, and by choosing the more specific welfare number rather than a general social science number, the document lands where readers interested in relief work will actually look.

Wooden ceilings 🔗

A book titled wooden ceilings forces a choice between a material, wood, and a building element, the ceiling. You could file it with timber and wood as a substance, or with building construction. The concrete focus of the work is the ceiling as a constructed feature, with wood serving only as the material it happens to be made from. The sensible decision is to class it with ceilings under building construction and treat wood as the secondary, material aspect. The object being built is the real subject; the substance is a property of it.

Atomic weight 🔗

Now take a work on atomic weight. The temptation is to scatter it across general physics or general science, but atomic weight is a defined topic within chemistry. Specificity tells you to choose the precise chemistry number that addresses atomic and molecular properties rather than a broad science number padded with zeros. By combining the specificity rule with the rule of zeros, you reject the vague, zero-heavy general number and select the tighter, more meaningful one. The book then sits among related chemistry titles rather than drifting in a general science section.

The order in which the rules apply 🔗

These guidelines are not a flat list; they form a loose hierarchy. The rule of application is checked first, because a work on interrelated subjects belongs with the subject acted upon, and this overrides other preferences. After that, specificity guides you toward the narrowest accurate number. The rule of zeros then helps you decide between competing numbers at the same level by favouring real subject digits over filler. Where two subjects are genuinely balanced and nothing else decides the matter, the first-of-two rule breaks the tie by schedule order. Above all of this sit any explicit instructions printed in the schedules themselves, which always take priority over the general rules.

Applying these principles in everyday classification work 🔗

For students learning practical classification, the value of these rules lies in turning a vague judgement into a repeatable procedure. When you face a document with no obvious single number, work through a short mental checklist. First, identify the true subject and the discipline, since the same subject can appear under several disciplines depending on how it is treated. Second, ask whether one subject is acting on another, and if so, follow the thing acted upon. Third, reach for the most specific number that still accurately describes the work. Fourth, when two numbers compete at the same point, prefer the one with fewer zeros. Finally, if the subjects are equal and nothing else decides it, take the number that comes first in the schedules.

A few habits make this easier over time. Always read the schedule notes before settling on a number, because a printed instruction can quietly overrule every general rule. Keep a record of borderline decisions so your library stays consistent across classifiers and across years. And remember that the aim is to serve the reader: the number that best gathers a document with its true neighbours is almost always the right one. These same instincts carry over to UDC and, with adaptation, to faceted schemes like Colon Classification, where the citation order of facets performs a similar job of fixing priority.

Mastering these rules will not remove every hard case, since classification will always involve a measure of judgement. What the rules give you is a shared language and a defensible logic, so that your choices hold up to scrutiny and your shelves stay coherent.

What do you think? When you imagine a book that could fit two equally specific numbers, would you trust the first-of-two rule, or would you lean on your own sense of where readers expect to find it? And in your view, does preferring the concrete over the abstract always serve the reader well, or can you picture a subject where the abstract treatment is what people are really searching for?

References

1. https://www.oclc.org/content/dam/oclc/dewey/versions/print/intro.pdf
2. https://ebooks.inflibnet.ac.in/lisp2/chapter/introduction-to-major-classification-systems-structure-and-features-of-cc/
3. https://www.planfl.org/handouts/20220823fay4.pdf
4. https://en.wikipedia.org/wiki/Concrete_number
5. https://ddc.typepad.com/025431/2012/09/the-rule-of-application.html
6. https://help.oclc.org/Librarian_Toolbox/OCLC_glossaries/Dewey_Decimal_Classification_glossary
7. https://www.britannica.com/science/Colon-Classification