Early in my indexing career I indexed a gardening book. In trying to follow the principle of specificity, I decided to create an entry for each tool which was discussed within a brief section on gardening tools. In her review, the client, who was also the managing editor for the book, commented that a single entry for gardening tools would have been sufficient. I can’t remember now, but she might have changed the index to reflect her point of view.

Regardless of who was right (I am willing to believe she was), this feedback alerted me to the issue of depth in an index. By depth I mean that there can be different layers in an index, which often reflects the types of information in the book, and which exists in a continuum from general or broad information to more and more detailed. The question is, which layers should be included in the index and which can be left out. 

Conceptualizing Depth

Let’s look at a couple of examples of how depth can manifest. 

The first, as mentioned, is to consider the different types of information in the text. For example, a book on transportation might have the following layers:

• transportation (metatopic)
• motor regulations (main topic)
• taxies (subtopic)
• Yellow Cab Company (example)
• Jean the Taxi Driver (example/passing mention)

Each of these provides a different point of view and point of entry to the subject. An index should probably have entries for most, if not all, of these levels. 

Depth can also be shown through the use of subheadings and cross-references. In the following example, the subheading and cross-reference reveal a set of relationships, at least one of which is clearly hierarchical. 

• motor regulations: taxies. See also Yellow Cab Company

Finding the Sweet Spot

The opposite of depth is a flat index, which I think is rarely helpful for the reader. By flat, I mean an index with no or very few subheadings, which means relationships are not indicated. Most terms also seem to come from the same level, likely on the upper, conceptual end, and subtopics and examples may not be present. While such an index gives a broad sense of the book, it does not facilitate more precise searching and much of the text remains opaque or hidden.

Yet, it may not be appropriate to include every level in the index. For the gardening index I wrote, the client clearly thought that having individual entries for each tool was a step too deep. It was enough to have a single, more general entry for tools. Going deep in this instance likely bloated the index. 

We can also question if the uppermost level—the metatopic—should be included in the index, or at least whether the metatopic entry should be prominent. The metatopic entry can often serve as a signpost to direct readers through the index, and while this can be helpful in many books it is not true for all. To use gardening again as an example, many gardening books clearly state the metatopic on the front cover and the reader likely has a clear idea of what they want to find. Signposting from the metatopic, while possible, is probably not necessary, and so that top level can be eliminated or reduced. 

In a way, depth in an index is about finding the sweet spot between too general and too detailed. It is about finding the right amount of detail with which to communicate to the reader. 

Tips for Choosing Appropriate Depth

So, how do we decide which layers to include?

One way is to consider the audience. I think most scholarly indexes should be deep, reflecting the level of thought that scholars put into their work. Readers of trade books, on the other hand, may not want to spend as much time searching in the index. For them, more general entries may be sufficient, leaving the reader to discover the finer details on their own.

We can also consider how the item is discussed in the text, as well as how the audience might perceive the information. If we can understand how that piece relates to its context, it can be easier to see its relative importance.

A last consideration is space. If there is a page or line limit for the index, then a decision will have to made about which entries to keep and which to cut. In this situation, I usually skew towards a shallower index, cutting out more specific detail so that at least the reader can get a broad overview of the book from the index.

In the end, decisions about depth will probably involve all three of these aspects. Trade-offs may have to be made, and clients or other indexers may have different opinion about what is appropriate. But I think these decisions will be easier if we can see and understand depth in an index, including how we can manipulate depth and how depth affects the index and the readers’ experience.