Charles Ellicott Commentary 1 Corinthians 14:9

"So also ye, unless ye utter by the tongue speech easy to understood, how shall it be known what is spoken? for ye will be speaking into the air." — 1 Corinthians 14:9 (ASV)

So likewise ye.—This is not the application of the preceding, but the introduction of a third illustration, namely, the varieties of human language. The “tongue” here is simply the actual organ of speech, distinguished in Greek by the insertion of the article from “tongues” which flow from the spiritual gift. If a person does not use words that those they are speaking to understand, it is useless; such words pass as sounds into the air and are useless.