Prayer and Religious Articles: Prayer and Blessings: The Tachanun:
What is the origin of Tachanun?
Basically, Tachanun is a prayer said by an individual after the Shemoneh Esreh. Such a prayer was known in Talmudic times, when it was called "nefilas apayim" because it was said lying face down. By the time of the Geonim this had been modified to lowering the head onto the arm. In the classical Siddurim (of R. Amram Gaon, R. Saadia Gaon, Maimonides, etc.) there is considerable variety in the wording given for the Tachanun prayers. They didn't achieve their present forms until the spread of printed Siddurim.
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