1 Corinthians 14:9
So also ye, unless with the tongue you speak an intelligible discourse, how shall it be known what is spoken? for ye will speak to the air.
1 Corinthians 9:26
I therefore so run not as uncertainly; so do I exercise with my fists, not as striking the air:
Treasury of Scripture Knowledge
Summary
So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
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Bible References
Easy
1 Corinthians 14:19
but in the church I had rather speak five words with my understanding, that I may instruct others also, than ten thousand words in an unknown tongue.
For
1 Corinthians 9:26
I therefore so run not as uncertainly; so do I exercise with my fists, not as striking the air: