In my last post I wrote about entries and arrays, which I described as the building blocks of an index. I defined an entry as “what this thing is + where to find it.” Today I’m going to expand on “what this thing is,” also known as main headings and subheadings.

The fundamental purpose of an index is to guide readers to the relevant information that the reader is searching for. To do that, the index needs to be clearly written, which begins with the first words that the reader sees. 

Main Headings

The main heading, also known simply as the heading, kicks off the entry. This is the first word or phrase that you see in an entry and array. It is typically a noun, and should be clear and concise. If a longer phrase is needed, the main heading should lead with the most important element. The main heading should also match how the term is used in the text, such as using the same spelling and capitalization. 

The heading should be reflective of both the text and the audience. Is the book discussing cars more generally or electric vehicles specifically? Or both? Will readers be reading this book to learn about the auto industry, new innovations, or specific brands and models? Or all of the above? To give another example, biblical characters such as Matthew, Silas, and Timothy probably don’t need a gloss clarifying their identity in a work of biblical studies to clarify their identities, but these names may be more unfamiliar to readers if they appear in other disciplines. 

Another consideration is whether or not to pluralize main headings. Should it be dog or dogs? Cantaloupe or cantaloupes? To start, be mindful of differences in nuance. Freedom is somewhat different from freedoms, for example. Otherwise, I tend to follow common usage. If a term is commonly pluralized, then I go ahead and make it plural in the index, which I think reads more naturally. 

To give a few examples of main headings:

Acts (biblical book)

Cleveland, Grover

electric vehicles

heat transfer

London (ON)

trade wars, retaliatory

Subheadings

For short arrays containing a handful of locators, a main heading is usually sufficient to specify what this thing is. But more specificity is often needed for topics with extensive discussion (usually when there are more than 6-10 locators) or if there are different aspects that readers would appreciate differentiated.

The subheading is placed after the main heading. Its purpose is to further clarify what this thing is. Because subheadings often differentiate references from one another, there are usually multiple subheadings per array.

Since the subheading is appended, there is more flexibility in how it can be phrased. Depending on the context, the subheading can be either a short word or phrase, or it can be longer and more descriptive.  In all cases, the relationship between the main heading and subheading should be clear. If possible, I try to also lead with the key word, which both affects how the subheadings are alphabetically sorted and, I think, makes it easier for readers to find the subheading as they scan the array. 

For example,

Acts (biblical book): authorship; within biblical canon; commentaries on; Paul within

Cleveland, Grover: first presidency; free-silver issue; legislative achievements; private life between presidential terms; second presidency

heat transfer: conduction; convection

Effective headings and subheadings connect readers to the text. Main headings are the point at which readers encounter the index, and readers should not need to guess what this thing is. The same is true for subheadings, if the reader decides to read further into the array. 

Tell the reader what they need to know. Be specific and concise. Do this, and your index will be well on its way to being excellent.