If the index structure can be likened to a skeleton, and the information that the index seeks to convey the lifeblood, then perhaps all the small details that shape and bind an index together can be considered the muscle and sinew. It is this attention to detail that can elevate an index. One specific type of detail, which I have been considering and using in some of my own recent indexes, are double-barrelled terms. 

By double-barrelled terms, I mean terms that bring together similar and related concepts into a single heading or subheading. The different components are usually connected by an and or a forward slash.

At the main heading level, I think this works best for concepts that are clearly related in some way. These could also be variations of a word, and if the terms would sort side-by-side anyway. Think architects and architecture, or canoes and canoeing. These terms are not quite synonymous, so if going the double-barrelled route, make sure that there is overlap in how these terms are discussed in the text. I find it also helps if each term would be a relatively small entry on its own. Otherwise, if the terms are used in distinct ways or would form substantial entries by themselves, then two separate entries are probably the better option.

Double-barrelled terms can also work as subheadings. The first situation would be the same as discussed above for main headings. The second situation (which does not work as well for main headings) is to link together related events. For example, in a family history I recently indexed I had the following subheadings: cancer and death; wedding and married life.

I think this works because it shows a clear chronological progression. This also works when the two discussions happen on the same page or across a small span of pages. Separate subheadings would have simply duplicated the information. As with main headings, it also helps if there are only a few locators to gather together. If there are a lot of locators—say an entire chapter or section each for the wedding and then married life, then separate subheadings make more sense.

If you need to shorten the index, double-barrelled terms can also be another tool you can use for merging entries and cutting down on duplication. It will probably not make a big difference, but can still be useful if just a few lines need to be cut or if used with other methods for shortening. 

Otherwise, I think elegance is the main reason for using this technique. Double-barrelled headings and subheadings can bring terms together in a way that is less didactic, and can provide a more nuanced understanding of the book than through standalone entries. Double-barrelled terms are not always applicable, but are useful to keep in mind for the right book.

What do you think of double-barrelled entries? Do you use them in your own work?