The distinction between a main entry, a subentry and sometimes a further subdivision is one of the most important decisions in professional indexing. The right hierarchy turns a large collection of discussions into a clear, organised search tool.
A main entry generally suits a subject that stands on its own: a broad or central concept readers may look up directly. A subentry describes a particular aspect of that subject. A further subdivision is useful when a subentry itself contains several distinct groups of discussions.
For example, if a book discusses many aspects of Shabbat, it is not necessarily helpful to create dozens of separate main entries for every detail. A main entry for Shabbat may contain accepting Shabbat, kiddush, prohibited work and Shabbat meals. If accepting Shabbat includes many different kinds of discussions, it may need further subdivisions.
A practical principle is: when several discussions naturally belong to the same family of subjects, bring them under a shared heading; when that group becomes too long or varied, consider dividing it further.
But not every small detail deserves another level. A hierarchy that is too deep can be as burdensome as a flat index. If readers must pass through four or five layers to reach a discussion, the structure starts working against its purpose. Two or three levels are enough for most books; deeper structures should be reserved for material that truly needs them.
When does a subject deserve a main entry?
Several practical signs suggest that a topic should stand on its own:
- It appears in many places in the book.
- It contains several kinds of discussion.
- Readers are likely to look it up directly by name.
- It has independent meaning outside a broader subject.
- Placing it under another entry would make it difficult to find.
If a point mainly makes sense as part of a larger subject, a subentry is usually more appropriate.
When should a subentry be divided?
This is a delicate editorial judgement. A subentry may begin with a few references and gradually accumulate ten, twenty or more discussions. The editor must then decide whether it remains one clear group or now contains several different subjects.
If a single subentry includes definitions, timing, methods, exceptions and disputes, further divisions may help. Instead of one long list, readers get a structure that leads them quickly to the discussion they need.
Different books need different hierarchies
No fixed hierarchy suits every book. A central subject in one work may receive only a passing mention in another. Indexing is therefore a process of understanding the book, rather than mechanically dividing headings.
At every stage, the editor should ask: How will readers search for this subject? Which broader concept does it belong to? Should readers reach it directly, or through another entry?
Sometimes a cross-reference should lead from an alternative term to the entry where the material has been gathered, allowing readers to find it even when they search under different wording.
A good index is a map of the book. Main entries show the broad areas, subentries organise the topics within them, and further subdivisions lead to the precise discussion without losing the overall picture.
The better the hierarchy, the easier the index is to scan, understand and use.