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."
- Davis, Rabbi Avrohom (2007). Kav haYashar. Monsey, NY: Eastern Book Press. pp. xxvii.