The words tach, tack sound the same but have different meanings and spellings. Why do tach, tack sound the same even though they are completely different words?
The answer is simple: tach, tack are homophones of the English language.
Informal A tachometer.
A short, light nail with a sharp point and a flat head.
Nautical A rope for holding down the weather clew of a course.
Nautical A rope for hauling the outer lower corner of a studdingsail to the boom.
Nautical The part of a sail, such as the weather clew of a course, to which this rope is fastened.
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.
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").