Mishnah Berurah משנה ברורה
The Chafetz Chaim's commentary on Orach Chaim, the practical handbook of daily law for the Ashkenazi world.
By the end of the nineteenth century hundreds of commentaries and three centuries of dispute had accumulated around Orach Chaim — prayer, Sabbath, festivals: the most everyday part of the law had become the most tangled. Rabbi Yisrael Meir haKohen of Radin, known by his first book as the Chafetz Chaim, spent some twenty-six years drawing it together.
The Mishnah Berurah is practical in design: the main text explains the law in plain language, Shaar haTziyun gives sources below, and Beur Halacha treats hard cases at length. The author does not rule by preference but weighs the later authorities and says what to do.
Published in six volumes between 5644 and 5667, it was accepted almost everywhere — a rare case of a private scholar's work becoming the standard.