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.