The words key, quay sound the same but have different meanings and spellings. Why do key, quay sound the same even though they are completely different words?
The answer is simple: key, quay are homophones of the English language.
A notched and grooved, usually metal implement that is turned to open or close a lock.
A similar device used for opening or winding: the key of a clock; a can that has a key attached.
A means of access, control, or possession.
A vital, crucial element.
A wharf or reinforced bank where ships are loaded or unloaded.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.