The words franc, frank sound the same but have different meanings and spellings. Why do franc, frank sound the same even though they are completely different words?
The answer is simple: franc, frank are homophones of the English language.
See Table at currency.
The primary unit of currency in Belgium, France, Luxembourg, and Monaco before the adoption of the euro.
A franked piece of mail.
Informal A frankfurter.
Open and sincere in expression; straightforward: made several frank remarks about the quality of their work.
Clearly manifest; evident: frank enjoyment.
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.