1 Corinthians 16:21

The salutation of me Paul with mine own hand.

This was Paul's authentication of the epistle, his signature. Paul's letters were usually written by a secretary, an amanuensis, probably Sosthenes in the case of this epistle (1 Corinthians 1:1). Tertius wrote Romans (Romans 16:22); and Paul also wrote the salutation and signature of 2Thessalonians (2 Thessalonians 3:17), indicating that an unnamed amanuensis wrote that epistle also.