A harrow is a tool drawn by horses and used to plow the land. The two harrows that God’s messenger, Grace gives Piers Plowman symbolize the Old Testament and the New Testament. The harrows are to be pulled by four horses (the four Fathers of the Western Church) to further prepare the land for planting after it has been plowed by the four oxen (the four Gospels). In this way, the Church Fathers, the Gospels, and the Old and New Testaments must all work together to establish the Church.
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Two Harrows Symbol Timeline in Piers Plowman
The timeline below shows where the symbol Two Harrows appears in Piers Plowman. The colored dots and icons indicate which themes are associated with that appearance.
Passus XIX
...Grace also gives Piers four horses, named Austin, Ambrose, Gregory, and Jerome, as well as two harrows , one old and one new.
(full context)