Answer:
Spelling or sound (depending if it is a grammar dictionary or a phonetic one)
Explanation:
A guide word is any of the terms at the head of the page in a dictionary indicating the alphabetically first and last words on the page, it helps find concepts faster, because it allows the reader to know what words are in that page, it is based on the spelling, in a regular dictionary, or in it's sound in a phonetic dictionary.
