"Answer Key" or "Answer Keys"?

I am a native English speaker working in a team of writers for whom English is their second language. Although their level of English is very high, I do a lot of editing. We create training and in some manuals we include an 'Answer Key'. I often see people use 'Answer Keys' as a heading for this section. I instinctively change that to 'Answer Key' even though there are 'keys' to multiple assignments included in that section. I can't explain why but I think it should always be singular, although 'key' used in other contexts can undoubtedly be plural. Can anyone shed light on if I am following a rule correctly or if it is just a personal feeling?

Solution 1:

It seems like the proper use is the singular Key, especially if it is a complete section.

This is similar to a Legend, which is defined as

a table on a map, chart, or the like, listing and explaining the symbols used.

where the whole table, which contains many pieces of information, is referred to in singular.

Keys are not what appear in this section, answers are. Rather the section is a Key.