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When the shin is representative of the intellectual dimension, the three lines stand for the three intellectual faculties of the Sefirot: the right line being Chochmah: the flash of an idea; the left line being Binah, understanding; and the center line Daat, application of knowledge.
There is a dimension of the emotions, or middot. Here the shin’s right line represents Chessed, kindness; the left line represents Gevurah, severity or discipline and the center line represents Tiferet, mercy or compassion.
It is the letter by which the Patriarchs communed with G-d.
Abraham is represented by the right line, Chessed (loving, kindness), as he personified absolute kindness, and good deeds and connection to others. Isaac is represented by the left line, Gevurah (discipline and severity), indicative of his being introspective and demanding of himself; concentrating on self refinement and intense prayer. Yaacov is the center line; this is Tiferet or harmony, because he took the qualities of Abraham and Isaac and synthesized them into mercy. Yaacov also represents Torah study, because the Torah blends the positive and negative commandments into a harmonious whole.
There is a suprarational dimension, Kesser, where the right line is the light of kindness; the left line is the light of justice and the center line is the light of mercy.
Form: it comprises three vertical lines representing three columns. This letter looks like a crown.
The letter shin has four forms. There is a shin with a dot above the right column, a shin with a dot above the left column, a shin with four columns instead of three and finally a silent shin.
When the dot is on the right, it emphasizes Chessed, the concept of kindness. When the dot is on the left column it emphasizes the aspect of judgment and severity and it is pronounced “SIN”.
The shin with four columns is found on the tefillin that is worn on the head. The Rebbe says that the four-lined shin is the shin of the LUCHOT, The Tables of the Ten Commandments. The four lines represent the awesomeness and holiness of the engraving of G-d’s word into physical stone. To visualize this, imagine the three lines of the shin etched into stone. If you focus on the stone that remains around the shin, there will be four columns.
The Rebbe also says that the four-pronged shin represents the four mothers: Sarah, Rebecca, Rachel and Leah. Like the Luchot, the teachings of our mothers are truly and lovingly engraved on our hearts and minds before birth. In contrast, the instruction of a father is inscribed, and easily erased, later in life and in an atmosphere of austerity and severity.
The silent shin: when two shins are together, only the first one is pronounced. They represent two of Leah’s sons, Yissachar, who studied Torah and Zevulun, who would conduct business for his brother to study.
Numerical value: 300.
Number three hundred represents each of the three columns; the shin stands for shuvah, penitence; it also represents the Day of Atonement-Yom Kippur.