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Possible Duplicate:
Double Yud as G-ds Name
In the siddur we often do not write out the four letter name of God (yud,kay, vav, kay) but instead write yud, yud. Why do we do this? Is this a different name of God (for example, can it be erased)? Why do we choose this letter combination as a stand in for the 4-letter name?
There's a yutorah shiur on sheimos (sorry I don't recall which off-hand) that mentions that printers sought a less-holy way to indicate yud-hey-vav-hey (so there would be less problems disposing of the papers), so they printed a double-yud with a horizontal bar on top that was supposed to signify a vav. Yud + yud + vav = 26 in gematria, the same as yud-hey-vav-hey.
To answer part of the question. The Rema in Yoreh Deah Siman 276 Sif 10 writes that the "Shem" that is written in Siddurim with "beis yudin v'achas al g'behen (2 yuds and one on top)" may be ereased if it's l'tzrich. The Shach in Sif-Katan 14 adds in the Piskei Maharei (as well as what the Darchei Moshe brings down) it's only if it's "l'tzrich gadol".
You see from here this was already before the time of the Rema that the Shem was written in siddurim like this.
