flatten out vs flatten

flatten out and flatten both are verbs.

Word	Noun	Adjective	Verb	Adverb
flatten out	No	No	Yes	No
flatten	No	No	Yes	No

As verbs, flatten out and flatten are synonyms defined as:

flatten out (verb)	flatten (verb)
become flat or flatter	become flat or flatter
	make flat or flatter
	lower the pitch of (musical notes)