Sefer Kav HaYashar The Just Measure Rabbi Tzvi Hirsch Kaidanover Kav ha-Yashar (lit. The Just Measure; קב הישר) authored by Rabbi Tzvi Hirsch Kaidanover (1648–1712), is one of the most popular works of musar literature of the last three hundred years. First published in 1705 in Frankfurt am Main, it has appeared in over 80 editions, in nearly every country around the world. The work was famous for uplifting the spirits of Jewish communities in Europe after the Chmelnitzki Massacres of 1648-1649. Moshe Idel has described Kav Ha-Yashar as an "ethical-kabbalistic collection of stories, moral guidance, and customs" which "reflects a deliberate effort to popularize Safedian Kabbalah by adopting a much more understandable style in Hebrew."Kav ha-yashar (the title means “an honest portion,” but the letters of the first word correspond to the number 102, which is the number of chapters in the book and the numerical value of Tsevi, his first name; the numerical value of the second word is equivalent to that of his second name, Hirsh) consists of “moral teachings and the fear of God and dreadful deeds,” as described on the first page of some of its editions; it was one of the most influential and widely circulated ethical works of the eighteenth century. Dozens of editions of the original Hebrew version have been printed, and Tsevi Hirsh also prepared a Yiddish version, which went through at least 10 editions. The book is based on the dualistic doctrine of the Zohar, which attributed palpable reality to both the world of sanctity and the world of impurity, on earth and in heaven. It divides all aspects of reality according to their affinity either with the sanctity connected to the divine, the angels, and the Garden of Eden or with the impurity connected to Satan, Lilith, evil spirits, plagues, and kelipot (“shells,” a kabbalistic term for the impurity surrounding holiness). This influential book was regarded as a guide to God-fearing piety and to preserving the norms of the community, framing them in the context of individual providence and the reckoning of sins and merits. Following motifs in kabbalistic literature, the book propounds a cultural, religious, and social system of normative versus deviant behavior, enforced by often terrifying sanctions that depend on a supernatural and invisible source.