The words chaise, shays sound the same but have different meanings and spellings. Why do chaise, shays sound the same even though they are completely different words?
The answer is simple: chaise, shays are homophones of the English language.
Any of various light open carriages, often with a collapsible hood, especially a two-wheeled carriage drawn by one horse.
A post chaise.
A chaise longue.
Plural form of shay.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.