The American Church Dictionary and Cycopedia
Sentences, the Opening
Short passages of Holy Scripture read at the beginning of Daily Morning and Evening Prayer, are so called, and are intended to strike the keynote of the service to follow. Originally the Daily Services began with the Lord's Prayer, but in 1552 the Sentences, with the Exhortation, Confession and absolution were prefixed to Morning Prayer; they were not placed in the Evening Prayer until 1661. In the last revision of the American Prayer-book additional Sentences were added and arranged to strike the keynote of the Church's great Festivals and Fasts, such as Christmas Day, Good Friday, Easter, etc.
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Miller, William James. Entry for 'Sentences, the Opening'. The American Church Dictionary and Cyclopedia. https://www.studylight.org/dictionaries/eng/acd/s/sentences-the-opening.html. 1901.