In the Torah
The text of the Torah is divided into roughly 150 sedarim though sources disagree on the exact number. Differing texts record 141, 154 or 167 sedarim
The division of the Torah into sedarim is related to the Palestinian Jewish tradition of a triennial cycle for Torah reading. The Babylonian Talmud states that it was the custom of Jews in Israel to read the Torah in a three year cycle.
In other parts of the Bible
Unlike the parashot (another subdivision of the biblical books in the Masoretic Text that is indicated by various spacing techniques), which are thematic divisions of the text, the divisions indicated by the sedarim is mostly quantitative. In Tiberian masoretic manuscripts it is noted in the margin.
The sedarim are seldom used in modern times. The Babylonian tradition of completing the Torah in an annual cycle became the dominant tradition, and eventually the Palestinian reading cycle ceased to be used altogether. This made the sedarim liturgically obsolete.
Additionally, the use of the chapter divisions of the Bible has made the sedarim unnecessary as structural divisions in the text.
In its sense as part of cyclical public reading of a biblical book, the term is also used to designate the Weekly Torah portion. In this sense it is often called sidra (plural sidrot) from the same root.
- Moses, Lionel E. "Is there an Authentic Triennial Cycle of Torah Readings?" p. 334
- Tractate Megillah 29b