What rule applies to this word desire?

It's difficult to determine what specific rule you're looking for without more context. Could you please provide more information?

For example, are you asking about:

* Spelling rules: Are you wondering about the "e" at the end of the word?

* Grammatical rules: Are you asking about how "desire" is used as a noun, verb, or adjective?

* Pronunciation rules: Are you asking about how to pronounce the word?

* Word formation rules: Are you asking about how "desire" is related to other words, like "desirable"?

Once you clarify your question, I can provide a more specific and helpful answer.