Shaar HaKolel is a sefer written by Rabbi Avraham Dovber Lavut, the Rebbe’s grandfather. The sefer is divided into many chapters and includes evidence and sources supporting the Chabad nusach of tefillah, which was established by the Alter Rebbe based on the Kabbalah of the Arizal.
The book is named Shaar HaKolel (The All-Inclusive Gate) according to the Chassidic tradition that there are twelve gates corresponding to the different nusachim of prayer, each one aligned with the spiritual root of one of the Twelve Tribes. However, there is one gate called Shaar HaKolel, which is suitable for every individual among the people of Israel.