Understanding Simple Synthesis in DDC: A Step-by-Step Guide

Open any edition of the Dewey Decimal Classification and you will quickly notice something: the schedules cannot possibly list a ready-made number for every subject a library collects. There are simply too many topics, and new ones appear every year. To handle this, the DDC lets classifiers construct numbers that are not printed anywhere in the schedules. The simplest form of this technique is called simple synthesis, and learning it is the first real step toward classifying documents with precision. This guide breaks down what simple synthesis is, how the “add to” instructions and base numbers actually work, and how to build a number from start to finish using a clear, worked example.

What is simple synthesis?
Why simple synthesis matters
How simple synthesis works
The base number
The “add to” instructions
The rules you must follow
Example: administration of secondary school libraries
Step 1: find the base number
Step 2: identify the aspect in Table 1
Step 3: synthesise the number
A second illustration
Key takeaways for classifiers

What is simple synthesis?

Synthesis in the DDC means building a class number by joining two or more pieces of notation, rather than picking a single number off the shelf. The DDC began as a largely enumerative scheme, meaning numbers for known subjects were given ready-made, almost like a “mark and park” system where you select a number and place it on the document. Over time, to cope with the rapid growth and increasing complexity of knowledge, the scheme added many provisions for number building, allowing deeper subject analysis than enumeration alone could offer.

Simple synthesis is the most basic version of this. In practice, it usually means taking a complete number from the schedules and attaching a standard subdivision from Table 1 to it. There is no complicated branching or multiple add tables involved, just one base and one addition. That is what makes it the natural starting point for students learning practical classification.

Why simple synthesis matters

Standard subdivisions represent recurring, non-subject aspects of a topic, such as its theory, its history, its management, or the form in which it is presented (a dictionary, a periodical, a directory). These aspects appear across thousands of subjects, so listing them again and again under every schedule number would make the scheme enormous. Instead, the tables in volume 1 of the DDC hold these common concepts once, offering a far more economical alternative to repeated enumeration. Simple synthesis is how you tap into that economy and give a document a number that reflects its real focus.

How simple synthesis works

Two ideas sit at the heart of the process: the base number and the “add to” instruction. Get these two right and the rest follows mechanically. One point worth fixing in your mind early: adding numbers in the DDC never means arithmetic addition. It means suffixing one piece of notation to the right of another.

The base number

The base number is the foundation you build on. According to the official DDC documentation, number building begins with a base number, which is always stated in the instruction note, and to which another number is added. For simple synthesis, the base is usually a complete subject number drawn straight from the schedules, for example a number for a particular type of library, a branch of science, or a region of literature.

The “add to” instructions

The DDC does not let you bolt notation together wherever you please. With one important exception, number building is initiated only upon instructions found in the schedules. The exception is the addition of standard subdivisions, which may take place almost anywhere unless there is an instruction to the contrary. This is precisely why simple synthesis is “simple”: because standard subdivisions from Table 1 can generally be added on your own initiative, without hunting for a specific add note.

It helps to know the wider picture. There are four sources of notation for building numbers: Table 1 standard subdivisions, Tables 2 to 6, other parts of the schedules, and the add tables printed within the schedules. Simple synthesis works almost entirely with the first of these. The more advanced sources require explicit instructions and produce what is sometimes called complex synthesis.

The rules you must follow

A few mechanical rules govern every synthesised number, and ignoring them is the most common source of student errors.

Insert one decimal point only. When adding to a number from the schedules, you place a decimal point between the third and fourth digits of the complete number. However the number is constructed, there is only ever one decimal point, and any decimal points belonging to the added notation are dropped.

Drop terminal zeros where required. Unless other instructions are given, the terminal zeros in a main class or division number are dropped before adding a standard subdivision. The DDC documentation gives the example of journals in mathematics: 510 loses its final zero and the form notation is added to give 510.5.

Use double zeros when single zeros are taken. Where the single zero is already used for special subdivisions of a number, standard subdivisions move to double zeros. This is often called the rule of zero, and it keeps the synthesised concept from clashing with an existing special meaning.

Example: administration of secondary school libraries

Theory becomes clear once you build a number yourself, so consider a document about the administration of secondary school libraries. The task is to express two things in one notation: the subject (secondary school libraries) and the aspect (administration, or management).

Step 1: find the base number

Begin in the schedules under class 027, which covers general libraries and information centres. Working down the hierarchy, 027.8 covers school libraries, and a further subdivision gives you the number for secondary school libraries: 027.8223. This complete schedule number is your base. Notice that the decimal point already sits in its correct position, between the third and fourth digits, so you will not need to add another.

Step 2: identify the aspect in Table 1

Administration is a recurring aspect, not a subject in its own right, so it belongs in Table 1. The standard subdivision for management is -068. Because standard subdivisions can be added without a special instruction, you may attach this to your base number directly.

Step 3: synthesise the number

Suffix the standard subdivision -068 to the base number 027.8223. There are no terminal zeros to drop, and you do not introduce a second decimal point. The result is 027.8223068, a number that did not appear anywhere in the schedules yet now precisely expresses “the administration of secondary school libraries.” That is simple synthesis in action: one base, one addition, one decimal point.

A second illustration

The same logic powers countless other numbers. The official introduction to the DDC shows that reading instruction in the primary schools of Australia is classed as 372.40994: the base 372.4 (reading instruction in primary schools), plus -09 (history, geographic treatment) from Table 1, plus 94 (Australia) from Table 2. Once you can see the seams in a built number, the whole scheme starts to feel transparent.

Key takeaways for classifiers

Simple synthesis rewards habit and care more than memorisation. A few practical pointers will keep your numbers valid.

Always confirm the base. Build only from a complete, correct schedule number. A wrong base guarantees a wrong final number, no matter how neat your addition is.

Respect the instructions. Standard subdivisions are the main thing you may add on your own. Notation from Tables 2 to 6 and from other parts of the schedule should be added only when an explicit instruction tells you to, a point stressed in the teaching material from IGNOU. Trying to add notation without authority is how invalid numbers are born.

Watch the zeros. Decide consciously whether terminal zeros should be dropped, and whether the rule of zero forces you to double-zero, before you commit to a final number.

Do not over-build. You should not, as a rule, add one standard subdivision to another. When more than one aspect competes, the DDC provides a table of preference, though the rule of application can override it, as when teaching financial management in hospital administration is classed at 362.110681 rather than at a competing number. Knowing these priority rules prevents you from stacking notation incorrectly.

Check that the built number reads correctly. Translate your finished number back into words. If “027.8223068” genuinely means “administration of secondary school libraries,” you have synthesised it well. This habit catches most mistakes before they reach the catalogue.

Mastering simple synthesis gives you a dependable foundation. Once the base-plus-standard-subdivision pattern feels automatic, the more elaborate add tables and the geographic and language tables become far less intimidating, because the underlying logic is exactly the same.

What do you think? If your library received a document on the history of secondary school libraries instead of their administration, which standard subdivision would you reach for, and how would the final number change? And in your own collection, where do you think simple synthesis would add the most useful precision?

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References

PDF 📄

Table of Contents

What is simple synthesis? 🔗

Why simple synthesis matters 🔗

How simple synthesis works 🔗

The base number 🔗

The “add to” instructions 🔗

The rules you must follow 🔗

Example: administration of secondary school libraries 🔗

Step 1: find the base number 🔗

Step 2: identify the aspect in Table 1 🔗

Step 3: synthesise the number 🔗

A second illustration 🔗

Key takeaways for classifiers 🔗