Don't judge a book by its cover is the saying that appearances tell you little about what is inside. As they say, don't judge a book by its cover.
It works as a whole sentence and is usually said in defence of somebody or something plain-looking. The saying is about a hundred and fifty years old in this form.
1phrasedon't judge a book by its coverThe saying: appearances tell you little about what is inside.
“The shop looks awful, but don't judge a book by its cover.”
don't judge a book by its cover is a saying.
Also accepted in games: never judge a book by its cover, judge a book by its cover
Practised in Idioms