Rabbi Menachem Mendel, known as the "Tzemach Tzedek," was the third Rebbe of Chabad, and the great-grandfather and namesake of the Rebbe. (Photo: JEM) http://www.chabad.org/news/article_cdo/aid/2529070/sc/fb/jewish/19th-Century-Documents-Uncover-Origins-of-the-Rebbes-Hereditary-Honor.htm