Sorting and format are like twins. They both have to do with how the index is arranged. But whereas format is more about the visual aspect of the index—how the index appears on the page—sorting is about how entries and arrays are arranged one after the other.

As I like to emphasize, indexes need to be easy to use in order to be effective. This also applies to sorting. Arrays and entries need to be organized in a way that meets the reader’s expectations, so that the reader can seamlessly being searching.

For the most part, this means alphabetical sort. The index begins with A and ends with Z. This applies to main headings, stacking arrays one after the other. This also applies to sorting subheadings within an array.

The alternative to alphabetical sort is to leave entries unsorted, either in the order in which they are written or randomly thrown together. Either way, with no discernible pattern, the index would be very difficult to search. Alternatively, the index could be organized in page order (I’ve occasionally seen subheadings arranged in page order), but page order is not quick to scan and and the reader may just as well flip through the book. So, alphabetical sort has emerged as the universal standard for all languages which use an alphabet. 

A few pointers on sorting:

• Alphabetical sort is either letter-by-letter or word-by-word. In letter-by-letter, spaces between words are ignored, whereas in word-by-word alphabetization begins again after each space. For both, alphabetization also ends and begins again after punctuation. In my experience, letter-by-letter sorting is more common, though the Chicago Manual of Style recently changed their recommendation to word-by-word (as I discuss here). So far the publishers I work with are cautious about implementing this change, but we may be in the midst of an industry-wide shift.
• Sorting is typically according to the first significant word in the heading or subheading. Prepositions, such as A, An, And, and The, are usually ignored. At the main heading level, the preposition, as in The Metals Company, can either be ignored when sorting or the name can be inverted: Metals Company, The. In subheadings, where prepositions are more common within phrases, prepositions are simply ignored. 
• Because sorting is based on the first significant word, pay attention to term selection, especially in subheadings. Try to lead with the key element, so that readers will see the key term first as they scan the array and so that the subheading will sort to where readers expect. Leading with a less important term tends to bury the subheading, making it difficult for the key term to stand out. That said, subheadings should also read naturally, and trade-offs may need to be made between readability and sorting.
• Entries beginning with a numeral can be handled in one of two ways. Either sort the entries to the top of the index or array, with all the numbers gathered together and sorted in numerical order, or sort the number as if spelled out. For example, 33 Acres Brewing Company would be sorted as if spelled Thirty-Three. For the most part, I think numbers should be sorted as if spelled out, especially if there are only a handful of such entries and there isn’t anything that connects them. The two main exceptions, for me, are dates and military units. Sometimes, in subheadings, it is helpful to show a chronology, such as a series of elections (for example, 2019 federal election, 2021 federal election, 2025 federal election, etc…). For these, keeping the dates together, at the top of the array, clearly shows the chronological progression. For military units, such as 1^st Battalion, 3^rd Brigade, 4^th Army, etc…, these arrays are all the same type of entry (military units), and readers are likely familiar with the convention of numbered units, and so it makes sense to sort together at the top of the index, before the A section.
• Entries beginning with a symbol, such as chemical or mathematical symbols, can, similar to numbers, be either sorted together at the top of the index or as if spelled out. Be careful, however, as different people may pronounce or spell these symbols differently. Probably best to consult with the client on how they think readers will search. 
• If absolutely necessary, entries and arrays can also be force-sorted. This means deliberately placing an entry outside of the normal alphabetical sort. For example, placing a subheading for “overview” at the top of an array. Force-sorting should be done sparingly, to avoid confusing readers. But sometimes alphabetical sort isn’t the most intuitive approach for a particular array or subheading, and a small adjustment should be made. 

Sorting is about how arrays and entries—main headings and subheadings—are organized. Ideally, the sorting simply works and readers don’t really notice because they are too busy searching. Alphabetical sort is either letter-by-letter or word-by-word. Word-by-word is CMS’s current recommendation. Whichever you choose, be consistent.

The main change in regards to indexing is 15.66, which states:

I think this change makes sense.

I personally most notice the difference in sorting when indexing Asian studies books, where I tend to see a lot of surnames like Chen, Kim, and Liu. Being so short, these names often get mixed up with other headings when sorted letter-by-letter, whereas I think the index is easier to scan if all of the surnames are sorted together. I’ve also received instructions from a scholarly press to sort the index letter-by-letter except for the names, which the press wants force-sorted word-by-word. Which begs the question: why not sort the entire index word-by-word?

For example, here is a comparison of letter-by-letter compared to word-by-word.

The word-by-word sorting, for me, is a lot easier to scan and parse when like surnames are grouped together, and when names are sorted together above other terms. It makes me confident that I am seeing all of the names present, rather than being concerned that I am missing a name that is buried below.

Also note that the Liu names are sorted according to the clarified 15.85, which states:

This also makes a lot of sense and has been my practice for a long time. By ignoring the comma, the second portion of the name is treated equally for all names, whereas if the comma is taken into account, all the names with commas sort to the top and may cause some names to appear out of order. For example,

For another interesting comparison, as a colleague pointed out, try looking for the sorting differences in the indexes between the 17th and 18th editions of the CMOS. And if you’d like to see a full list of the changes to the indexing chapter in CMOS 18,see here.

So will I now unilaterally switch to word-by-word sorting for all of my clients who request that the index follows CMOS? I don’t think so, unless I think that the index will really benefit. I think it is better if I first ask my clients if they want to change, so we are both on the same page and I am not springing a surprise on them. And, to be honest, for most indexes I don’t think that the difference between word-by-word and letter-by-letter sorting will be that noticeable.

This brings me to my larger point, which is that the mechanics of a well written index should be invisible to the user. I doubt that any reader will browse the index and think, “I wonder what the alphabetical sort is?” That is not the reader’s concern. What the reader cares about is quickly finding information.

To facilitate finding information, every aspect of the index needs to work together. This includes the sorting, the structure, term selection, phrasing, and cross-references. When it works, the reader shouldn’t notice how the index works because the reader is too busy digging into the book. When the index does not work—that is the point when the reader is pulled out of the index and is frustrated at their inability to access the information they want. The reader may not be able to articulate whythe index is not working, but something about the contents and mechanics of the index is wrong.

Bringing this back to sorting, for many indexes the difference will be negligible between letter-by-letter and word-by-word sorting. As CMOS states, they will accept either in a well-prepared index. For other books, like for me with Asian studies titles, the difference will be more pronounced.

When indexing, pay attention to when the difference matters. Make decisions based on what will make the user experience the most seamless. Pay attention to how the different elements of the index fit together. Striving to make the index invisible may be an odd way to think about indexing, but to be invisible means that the index works, which is what we ultimately want for our readers.