When Peter defends the disciples at Pentecost, his first argument is the clock: these people are not drunk, it is only the third hour of the day. The verse fixed the hour in Christian memory, and when the early church built its scaffold of daily prayer times it hung Pentecost on the third hour peg. Tertullian, writing around the year 200, already lists the third, sixth and ninth hours as the apostolic times of prayer, and the church orders of the third century commend them to all believers, each with its scriptural warrant. Terce's warrant proved the most fruitful.
The office of the third hour became the office of the Holy Spirit: its ancient hymns ask the Spirit to come as once at this hour, the ninth-century Veni Creator Spiritus found a natural home there, and medieval custom preferred Terce as the hour before which the principal Mass of the day was sung and at which ordinations were set, so that the invocation of the Spirit in the office flowed directly into the sacramental action that followed. The service itself has always been deliberately small: a hymn, a block of three psalms under one antiphon, a short reading, a versicle and the concluding prayer, a shape shared with Sext and None. The little hours ask nothing heroic; they assume a person at work, and offer a few minutes' interruption in which the work is reconnected to its source. In that modesty lies their durability: they are the office's answer to the ordinary mid-morning.