Kav ha-Yashar (lit. The Just Measure; קב הישר), authored by Rabbi Tzvi Hirsch Kaidanover (1648^{[citation needed]}–1712; Rabbi at Frankfurt, son of Aaron Samuel Kaidanover), is an "ethical-kabbalistic collection of stories, moral guidance, and customs",^[1] and one of the most popular^[2] works of musar literature of the last 300 years. It serves as a guide to "God-fearing piety and to preserving the norms of the community", framing its teachings in the context of "individual providence and the reckoning of sins and merits".^[3] The title of the work alludes to the number of chapters it contains (קב = 102), as well as the author's name (הישר = הירש).^[4]

Kav ha-Yashar was known for uplifting the spirits of Jewish communities in Europe after the Chmelnitzki Massacres of 1648-1649.^[2] First published in 1705 in Frankfurt am Main, it has appeared in over 80 editions, in nearly every country in the world with a Jewish community. Kaidanover also prepared a Yiddish version, which went through at least 10 editions.^[3]

The work draws on "Yesod Yosef", a mussar work infused with the Kabbalistic teachings of the Ari written by Kaidanover's teacher Yosef Yoske of Dubno; ^[5] and Kav ha-Yashar then constitutes "a deliberate effort to popularize Safedian Kabbalah by adopting a much more understandable style in Hebrew."^[1] Relatedly, it reflects, to some extent, a Kabbalistic dualism, dividing all aspects of reality "according to their affinity" either with the divine or with impurity.^[3]