Machzor מחזור

The word means "cycle": the machzor is the round of the year. Weekdays and the Sabbath are held by the siddur, but Rosh Hashanah, Yom Kippur and the three festivals need another volume—because there the piyutim are, and the piyutim are many.

Why they differ. The order of the prayers is one, but each community chose its own piyutim—and by these the machzorim diverge more than by anything else. The Machzor Vitry, compiled in Rashi's school, is Ashkenazi; Italy, Romania, Provence, Spain and Yemen each have their own. A man who opens a stranger's machzor knows the prayer and does not know half the book.

Why this matters for history. The machzor is the most exact indicator of kinship between communities: by it one sees where people came from, when, and through whose hands. The argument over whether to change one's rite on moving is still going on.

What is in it besides prayers. Directions for what to do; sometimes lists of destroyed communities to be remembered; and in the manuscript machzorim of Ashkenaz, drawings in the margins, from which historians learn how these people dressed.