flatten vs flatten out

flatten and flatten out both are verbs.

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

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

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