As a very brief introduction, cross-references are like signposts that redirect readers to a different array in the index. Cross-references can either be See references (“aubergine. See eggplant”) or See also references (“Brassica, 34, 56-57, 63. See also cabbage”).

From General to Specific

Perhaps the most common scenario is to direct readers from general information to specific information. This is especially true if the more specific array has subheadings. Instead of double-posting or sub-subheadings, redirecting readers can be more space efficient and easier to read.

One way to think about the levels of information in a book and index is as a cascade. Information flows down, from most broad to most specific. Cross-references usually move in that direction too. To give an example, a book I recently indexed is about the key chemical elements nitrogen, phosphorus, and potassium. There is a brief discussion about chemical elements as a whole, and so I’ve included cross-references from the general (elements, chemical) to the specific (nitrogen, phosphorus, and potassium). 

From Specific to General

Can cross-references go in the opposite direction? I don’t want to say never, because it can happen, but I think it is less common. My assumption is that broader terms may be easier to remember, and if readers want to find the broader term, they would have started there anyway. So while I often create cross-references from the general to specific, following the cascade of information, I usually don’t make those cross-references reciprocal.

One exception is if the general term is the preferred term. Going back to the book on nitrogen, phosphorus, and potassium, fertilizer is a significant subject. For the most part, the discussion is about mineral fertilizers (also known as synthetic fertilizers). But while the terms mineral and synthetic fertilizers are defined, they are not actually used a whole lot in the text. Rather, it is implied that “fertilizer” usually means mineral fertilizer. Similarly, I suspect most readers will first search for “fertilizer.” So while I do include main entries for mineral and synthetic fertilizers (mostly for their definitions), I primarily use those entries to cross-reference to fertilizer.

Reciprocal Cross-references

As I mentioned, I don’t often include reciprocal cross-references from specific to general terms. But reciprocal cross-references are important in their own right. I usually use them for related headings that have equal weight. 

By equal weight, I mean that they are at about the same level of granularity or importance. This can include synonyms. Both arrays contain subheadings and/or locators, and readers searching for one may reasonably be interested in the other.

For example, a reader searching for the term “liberty” may also be interested in “freedom.” Facebook, the social media platform, should probably be linked to its parent company, Meta, and vice versa. Terms which belong to the same category can also be linked together with reciprocal cross-references. For example, an index with arrays for electrons, neutrons, and protons could include cross-references between all three terms. 

Preferred Terms

Reciprocal cross-references work best when both arrays contain searchable information. If only one array contains locators, or if there is otherwise a clear preference for one term over others, then a See cross-reference is needed to direct readers to the preferred term.

Preferred terms can come about for a number of reasons. There may be enough locators to warrant subheadings, and so a preferred term is chosen to gather the discussion in one place. The author and text may also emphasize one term over another. Perhaps a certain term is more inclusive or better reflects current usage. Whatever the reason, a See cross-reference anticipates that readers might look elsewhere and points readers in the right direction. 

Cross-References versus Double-Posts

Synonymous terms can raise the question of whether it is better to double-post. If “feelings” and “emotions” are both used in the text, are used in the same way, and used about the same number of times each—they carry about equal weight—then why not double-post under both instead of using cross-references? 

Double-posts and cross-references are like indexing twins. They both serve to provide alternative entry points, and there are situations in which double-posts can be the better option. Instead of sending readers off in a new direction, readers can find the relevant locators right where they are. In the above example, there are times when I have double-posted something like “feelings and emotions” and “emotions and feelings.”

My general rule of thumb is to double-post if there are no subheadings and to use a cross-reference if there are subheadings. Subheadings take up more space, and so it is more space efficient to redirect.

The other reason to choose cross-references is if there is a preferred term. Synonymous and related terms do not always have equal weight. If the text clearly prefers a term, or if a term is clearly preferable for another reason, then that should be reflected in the index

I’ll stop here for now. I’ll write again in a couple of weeks to discuss general cross-references, cross-references from subheadings, and cross-references to and from acronyms.

For now, keep in mind that cross-references are a versatile and powerful tool for shaping how readers navigate the index. If you can map out the structure of the index, with all information in its place, cross-references are the signposts that points readers to where they need to be.

Cross-references are a type of locator, but instead of directing readers to the text, cross-references redirect readers to a location within the index. Riffing off the metaphor of an index as a map, I like to think of cross-references as signposts within the index that ensure readers find the arrays they want.

Cross-references come in two types, See and See also. See references tell the reader, “Good effort, but the information you want is actually over there,” while See also references indicate, “if you liked this, maybe you will also like that.”

How to Use

• Cross-references should always point to new information, which is to say, new locators that the reader hasn’t seen yet. If the array the cross-reference is pointing towards has identical locators, then it is a double-post, which is also good, but renders a cross-reference unnecessary. 
• Speaking of double-posts, cross-references and double-posts are kind of like cousins, complementary tools for making information accessible within the index. Double-posts are when identical information is placed in two or more locations in the index, such as both a main heading and subheading, to accommodate readers searching either way. They work best for shorter strings of locators that are easily duplicated. The downside to double-posts is that they can take up space, especially if subheadings are involved. In that case, a cross-reference is the quicker and more space efficient option. 
• Cross-references are usually from broader topics to more specific topics. For example, “computers. See also Apple; Microsoft.” The logic is that readers less clear on what they are looking for will likely begin with the broader term, whereas if they already have the specific in mind, they will skip the broader term and go straight to the specific. For those readers uncertain, the cross-reference prompts them that there is more to the topic that they may be interested in. Can cross-references also go in the opposite direction, from specific to broad? Yes, sometimes. I don’t want to create a hard-and-fast rule, but I usually find broad-to-specific to be more meaningful. Think carefully about which direction readers are likely to desire.
• Building upon broad-to-specific, cross-references can also be used if a subheading needs to be hived off to create its own array. Say “Apple” is originally a subheading under “computers,” but as the index is created it accumulates a dozen locators, which you decide should be broken down into subheadings. Instead of using sub-subheadings under “computers,” turn “Apple” into its own array and use a cross-reference to redirect readers. 
• Cross-references can also be reciprocal. Say the book contains two or more related, but not quite identical, concepts. Cross-references are a good way to link these arrays, keeping in mind that readers should find something new in each. 
• Cross-references can link synonymous terms. Say the book uses the terms “film industry” and “movie industry,” and you decide to make “film industry” the preferred term in the index. A cross-reference should be used to redirect readers who search for “movie industry.” A cross-reference from “Hollywood” may also be a good idea, if the book is specifically about the American film industry.
• Cross-references can also be generic. There may be a category of arrays that you want to point readers towards and there are too many to list. For example, in a work of US history that covers multiple presidencies, the following may be useful: “United States of America. See also specific presidents.” The assumption here is that readers will know the names of specific presidents, or will remember which are discussed in the book, and will be able to search accordingly. 

Formatting and Placement

Where to place the cross-reference is an interesting question. The cross-reference is usually placed at the end of the array, after the other locators and subheadings, if any. But cross-references can also be placed at the beginning, immediately after the main heading.

For example,

computers: economic advantages; history of; rare earth minerals and; semiconductors. See also Apple; Microsoft; TMSC

and 

computers. See also Apple; Microsoft; TMSC: economic advantages; history of; rare earth minerals and; semiconductors

Both approaches have their advantages. Placing cross-references at the end allows the reader to first peruse the array to see if they can find what they want. If not, the cross-references are ready to offer other suggestions. Placing cross-references at the beginning allows readers to quickly see if they actually want to be elsewhere, before they dig into the subheadings. Placing cross-references at the end is more common, and I think is what most readers and publishers expect. However, for reference documents, especially, and for indexes with very long arrays, placing the cross-references at the beginning can be helpful for an audience that wants to search quickly. 

Cross-references can also direct readers to specific subheadings. I rarely do this, but that may have to do with the types of books I tend to index. If you need to, the cross-reference can be phrased as either “statistics. See under economics” or “statistics. See also economics—statistics.”

Cross-references can also be attached to specific subheadings, instead of being gathered in a group at the beginning or end of the array. I didn’t use to do this, but the recent NISO indexing standard (ANSI/NISO Z39.4-2021) recommends doing so, which prompted me to give this more thought and to adjust my practice. There are two questions that guide where I place the cross-reference: 1) How specific is the cross-reference? Is it more connected to the main heading or to the subheading? 2) How long is the array? If short, I think readers will easily see the cross-references at the end. If long, then attaching the cross-reference to the subheading, if relevant, allows the reader to be redirected sooner. 

In terms of formatting, See and See also are typically upper case and in italics. The exception is if the heading being directed to is also in italics. For example, “Austen, Jane. See Northanger Abbey; Persuasion; Pride and Prejudice.” Multiple cross-references are separated by semicolons. Cross-references are preceded by a period, but, if placed at the end of an array, no closing punctuation is needed. Cross-references attached to subheadings are in lower case and may be placed in parentheses to better differentiate from the surrounding subheadings. For example, “literature: authorship, 34, 53, 122; figurative language, 45, 53 (see also metaphor; similes); poetry, 56-60, 132, 154; translation (see translation)“ 

Some indexes are thick with cross-references, an interlocking web redirecting readers. This may be due to the book using a lot of synonyms, or similar but different terms, or concepts for which the indexable term is not obvious. Other indexes contain just a handful of cross-references. Either way, the goal is to ensure that readers find the information they desire.