It’s time for your indexing lesson again. I bet you can’t wait.
The standard for book indexes is to use plural forms of countable nouns. So in an index for a book about the wildlife of Indonesia, you might find entries such as:
Bornean earless monitors, 17
oranguatans, 50–55, 59, 62
Sumatran tigers, 32–33
Even if the Bornean earless monitor is only mentioned once in the text, and it refers to a solitary individual lizard, the index entry will still be in the plural.
So here’s yet another way to spot an inexpertly-written index — singular nouns as index entries, such as:
table, 54–67, 70
The correct form is, of course:
tables, 54–67, 70
Aren’t you glad you know that now? You’re welcome.