Prayer for grace; said after the "'Amidah" of the morning ("shaḥarit") and afternoon ("minḥah") prayers on week-days. It is so called from the initial words of "Raḥum we-ḥannun" ("O Merciful and Gracious; I have sinned before Thee. O Lord, full of compassion, have mercy upon me and accept my supplications"). This is followed by Ps. vi., and then by "Wa-anaḥnu lo neda'". ("Neither know we what to do: but our eyes are upon Thee" [II Chron. xx. 12]) and several verses from the Psalms. On Mondays and Thursdays poetical verses are interpolated pleading for divine intervention, evidently composed during the Crusades and persecutions. These verses begin: "O Lord of Israel, turn from Thy fierce wrath. . . . Strangers say, 'There is no hope or expectation for thee.' Be gracious unto a people that trust in Thy Name. O Lord, spare us in Thy tender mercy, and deliver us not into the hands of the cruel. . . ." After every stanza the congregation answers with the refrain, "Yet, despite all this, We have not forgotten Thy Name: we beseech Thee, forget us not." On public fast-days is added "Shomer Yisrael" ("O Guardian of Israel, guard the remnant of Israel, and suffer not Israel to perish, who recite 'Shema' Yisrael,'" etc.). The modern minhag always couples this with the "Taḥanun." The verse "Wa-yomer Dawid" (II Sam. xxiv. 14), preceding the "Taḥanun," is a later addition, first mentioned by Jehiel Michael Epstein in his "Kiẓẓur Shelah" (p. 56, Amsterdam, 1701). Its insertion is due to the fact that it contains the words, "Let us fall," since the "Taḥanun" is known also as the "prostration" prayer, it being customary for the worshiper while reciting the prayer in silence to sit with the head resting on the arm and with the face downward. The head reclines on the left arm, unless the tefillin are adjusted on that arm,when the right is substituted. Prostration is not performed in a place where there is no Sefer Torah. The "Taḥanun" is entirely omitted in a case of mourning or of joy; nor is it said on Sabbath, holy days, or semiholy days.
The ceremony of prostration is derived from Moses, who "fell down before the Lord" (Deut. ix. 18), and Joshua, who "fell to the earth upon his face" (Josh. vii. 6). The custom is connected with the expression "mappilim taḥanunenu" (Dan. ix. 18), which means, literally, "we cause our supplications to fall." It is mentioned in the Talmud as being the practise of the Babylonian Jews: when Rab happened to be in Babylon on a public fast-day he noticed that all the people fell on their faces in supplication. He, however, refused to do so, becuase, it is explained, he either considered it a pagan custom or regarded it as not obligatory upon a distinguished personage (Meg. 22b). The efficacy of prostration is shown in the case of R. Eliezer b. Hyrcanus, who in that way brought about the death of R. Gamaliel ha-Nasi (B. M. 59b).
The Zohar connects Ps. xxv. with the prostration prayer (Zohar, section "Ba-Midbar," p. 120b). This psalm is also given in the Vitry Maḥzor (p. 70), and is substituted in the Sephardic ritual for Ps. vi. Maimonides ("Seder Tefillot") gives quite a different version of the "Taḥanun"; the fact is that there was no fixed formula for this prayer up to the fourteenth century (see Ṭur Oraḥ Ḥayyim, § 131); and even as late as the sixteenth century it was composed in various forms to suit different minhagim (Shulḥan 'Aruk, l.c.) See Adoration, Forms of.