Half-Hallel on Rosh Chodesh

The custom of reciting an abbreviated Hallel on Rosh Chodesh mornings and the last six days of Passover, omitting sections from two psalms (the opening verses of Psalms 115 and 116). Rooted in Talmudic-era practice, the formulation as a fixed community custom with a preceding blessing was crystallized by Babylonian Geonic ruling and spread into near-universal usage. Transmitted to the Sephardic world via the Rif and Rambam, and to Ashkenaz via Rav Amram Gaon's Siddur and the Tosafists; codified near-universally in Shulchan Aruch OC 422. Yemenite and some Sephardic communities omit the blessing, retaining the recitation as pure custom without a benediction.