A form of literary communication. There was an organized postal system as early as the Persian Empire (sixth century BCE), but it existed only for government business and not for private correspondence. Wealthy families used slaves or couriers, and common people relied on caravans, friends, or passing strangers who were traveling in the direction of the recipient of the letter. Since no OT document is written in letter form, it is surprising that the letter became the largest literary category in the NT; i.e., twenty-one of the twenty-seven documents are written in epistolary form. A partial explanation of the prominence of letters in the NT is that Christian leaders such as Paul employed letters as a means of instructing communities under their supervision when they could not be present in person. NT letters tend to be longer and somewhat more literary than other letters from the period. Paul intended that his letters be read aloud to his communities when they were at worship.