The words knee, nee sound the same but have different meanings and spellings. Why do knee, nee sound the same even though they are completely different words?
The answer is simple: knee, nee are homophones of the English language.
The joint between the thigh and the lower leg, formed by the articulation of the femur and the tibia and covered anteriorly by the patella.
The region of the leg that encloses and supports this joint.
An analogous joint or part of a leg of a quadruped vertebrate.
Something resembling the human knee, such as a bent piece of pipe.
Used when giving the maiden name of a woman.
Used when giving a former name. Originally known as.
no, used to express no as a quantity, i.e. not any, like German kein/Dutch geen/French rien. Compare with na.
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.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").