The Hebrew letter Is, equivalent in sound to I or Y. It is the initial letter of the word Jehovah, the Tetragrammaton, and hence was peculiarly sacred among the Talmudists. Basnage (book iii, chapter 13), while treating of the mysteries of the name Jehovah among the Jews, says of this letter: The yod in Jehovah is one of those things which eye hath not seen, but which has been concealed from all mankind. Its essence and matter are incomprehensible ; it is not lawful so much as to meditate upon it. Man may lawfully revolve his thoughts from one end of the heavens to the other, but he cannot approach that inaccessible light, that primitive existence, contained in the letter Yod and indeed the masters call the letter thought or idea, and prescribe no bounds to its efficacy. It was this letter which, flowing from the primitive light, gave being to emanations. It wearied itself by the way, but assumed a new vigor by the sense of the letter t which makes the second letter of the Ineffable Name. In Symbolic Freemasonry, the god has been replaced by the letter G. But in the advanced Degrees it is retained, and within a triangle, as in the illustration, constitutes the symbol of the Deity.
In Mackey's Encyclopedia of Freemasonry.