The Basics of Judaism: G-d & Torah: The Abbreviation of G-d's Name:
Why is G-ds Name abbreviated "Yud Yud" in Siddurim?
The Talmudic Encyclopedia, article "Azkaros", gives three explanations for the use of three YUDs (in a triangle) to represent G-d's Name: (1) They correspond to the YUDs in Shakai, Elokim, and Ehyeh (Midrash Lekach Tov, also known as Pesikta Zutrasa, on Ex.3:15); (2) they correspond to the YUDs of Yevarechecha, Yaer, and Yisa (ibid., Num.6:26); (3) their Gematria, 30, is equal to that of the tetragrammaton plus 4 for its four letters (Resp. Radvaz 1:206). See also Rema, Yoreh Deah 276:10, and Torah Shelemah, Bereishis 1:272. Since it's hard to print three YUDs in a triangle, nowadays only two YUDs are used.
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