Conversely, piece of mind is a faulty combination of the two previous expressions, and should never be used. The phrase peace of mind refers to the relaxed mental state you feel when you’re freed from all sources of worry or trouble. In that sense, it is very similar to the expression putting your mind at ease.

Piece of mind is not an accepted alternative for either phrase. The individual words in each phrase signal its meaning. Peace is a synonym of calmness, which is also the meaning of the phrase peace of mind. By using this trick, you should always know when to use each of these phrases.