Passing Mentions

Posted on: 11/06/2024

A group of women cyclists in a road race, dressed in colourful cycling gear and leaning over to negotiate a corner.
Women’s cycling road race by Bob Mical, used under a Creative Commons 2.0 licence []

One of the most frequent queries indexers get from clients is “Why is this term not in the index?” In this post, indexer Tanya Izzard explains what passing mentions are, why we sometimes leave them out, and how we make that decision.


Passing mentions (sometimes called passing references or minor mentions) are usually defined as terms or concepts that are mentioned incidentally in a text, but do not include worthwhile information about the term or concept themselves. Dealing with passing mentions is one of the substantial jobs of indexing, and something we often talk about among ourselves. When deciding whether to include something, we rely on two key questions:

  • would anyone using the index to this book look up this term?
  • if they did, would they find any useful information here?

If we can answer “no” to either of those questions, then it’s a good sign that the term or concept is a passing mention.

The exclusion of passing mentions is one of the things that distinguishes indexing from simple searching. Do a Ctrl-F search on a text, and you’ll get all the occurrences of your keyword. But they may not all be indexable, and your indexer may decide not to include them.

Choosing what to leave out

What are the factors that indexers take into account when deciding to treat a term or concept as a passing mention?

  • Relevance: is the term closely related to the topic of the book, or tangential? Is it likely to be of interest to the likely audience for the book? Would they expect to see it in the index?
  • Significance: how much information is included? Significance can sometimes outweigh relevance: if there is substantial information here, we might choose to put the term in. This is the sort of query we might choose to refer to clients, too.
  • Ease of access: indexes need to direct the reader efficiently to the information they want. A pocket reference guide to trees I own includes lots of references to different types of trees in the introduction, but the index points the reader directly to the page that describes the tree. It can seem more generous to the reader to put in all the page numbers where a certain topic is mentioned, but if there is no additional information there, you are wasting the reader’s time.
  • Context of book and reader: reference books, manuals, cookbooks and similarly practical works need efficient, concise indexes. Indexes for books for children need to be similarly focused. Indexes for scholarly books may be more discursive but still need to point to relevant and significant mentions. For local and family history books, all the names are usually indexed; in other types of history, they may be less helpful.
  • Space: if space for the index is limited, the indexer will need to be more selective. Index entries noted as ‘nice to have’ can become passing mentions when space is tight.

Likely sources of passing mentions

Some parts of a book, and types of writing, are rich sources of potential passing mentions. Indexers will look at these carefully before deciding whether terms can be included. The examples below are all invented for the purposes of this post.

  • Introductory material: introductory chapters or paragraphs may summarise concepts discussed in more detail later on. Indexers will compare the full discussion with the summary to see whether introductory material needs to be indexed. To avoid redundant entries, many indexers will go back over their index entries for introductory material when editing.
  • Scene-setting: references to people or places that help to place the narrative of a text are often passing mentions. For example, from a history of textile production:

In the era of Queen Victoria, the weaving industry was undergoing significant change.

Queen Victoria is unlikely to be indexable here: she’s being used as a synonym to indicate the nineteenth century, and is almost certainly a passing mention.

  • Signposting: writers use various types of signposting to help the reader through their books. Flash-forwards and callbacks alert the reader to what’s coming, and reinforce what’s already been said. A book about kitchen equipment might include signposting like:

In the next chapter I will explain how to make mayonnaise using a stick blender.

Index entries for this flash-forward would be very unhelpful here; what we want in the index is the page from the next chapter with the stick blender mayonnaise method. Similarly, for callback signposting like:

Earlier, we learned how to make mayonnaise with a stick blender. Now, let’s explore how to make it with a food processer.

I would index mayonnaise on this page, because the method is about to be described, but indexing stick blender here would waste the reader’s time.

  • Lists of examples: these alert the indexer’s attention to a possible set of passing mentions, especially if preceded by “such as” or “including”,. Often lists like these are in introductory material:

Many plants are suitable for shady gardens, including ferns, hostas, hellebores and bergenias.

Are the plants named here indexable? It depends. If there is much more substantial discussion later on, then there’s no need for an index entry for this page. If this is the only mention and the discussion is very relevant to the book and its readers, indexers might consider including them, depending on space. But the use of “including” tells us that this is not a complete list, which may compromise its usefulness to the reader.

  • Repetitions: this can especially be an issue in multi-author works, or reference works where the reader is not likely to read the book from start to finish. If every chapter of a book of essays starts with a potted biography of the subject, it is probably not helpful to index them all. But we will need to evaluate them all and decide whether they have useful additional information that merits inclusion in the index.
  • Allusions and digressions: again, these may or may not be indexable and the indexer will weigh up their usefulness as index terms. For example:

At Tottenham Hotspur, Smith’s autocratic management style earned him the nickname ‘Little Napoleon’ from his teammates.

Napoleon is not indexable here. However, especially in humorous books, allusions and digressions may be highly relevant, and therefore worth indexing. But in my imaginary kitchen equipment book, a digression into gardening tools probably won’t make it into the index, because readers are unlikely to look for it.

An index is often described as a map of the book, but the map is not the territory. A map that is overly cluttered with too much minor detail is harder to use. You might also think of an index as a shop window for a book: you don’t want every item you stock in your shop window, but you do want the important, significant items to be clearly visible to tempt your readers in. Excluding passing mentions can feel stingy, but it’s actually generous to the reader, giving them the gift of space and focus to find the really important information the book has to offer.

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