John Locke had a unique approach to indexing his commonplace notebooks. He would determine one or more keywords for an entry, then, in his index, find the first letter of the keyword, followed by the first vowel of the keyword. Then he would write the page number in the corresponding block.

If a word started with a vowel and only contained one vowel, the first letter would be used in both instances. For example, “owl” would map to (O-O). 

I would think this approach would work well if keywords were obvious, as you would need to know the keyword to be able to look up the page(s) referencing it.

A digital example of the system John Locked used to index his commonplace books.