Shin is the twenty-first letter and its value is three hundred. It has no final form. Its shape carries three branches, and Rabbi Akiva relies on them: "just as this shin has three branches."
For Sefer Yetzirah, shin is one of the three mother letters, alef, mem, shin, "a great secret, wondrous and hidden." From shin comes fire, from mem water, and air, alef, decides between them, like the pointer between the two pans of a scale. "He made the letter shin king over fire, He bound a crown to it," and formed through it the heavens in the world, heat in the year and the head in the soul; for "the heavens were created from fire." In the classification of sounds, shin belongs to the letters of the teeth, zayin, samekh, shin, reish, tzadi.
The Talmud reports the reading of the alphabet that children once gave in the study hall, which the Sages came to relate to Rabbi Yehoshua ben Levi. "Shin: sheker, falsehood; tav: emet, truth." Why are the letters of sheker adjacent, and those of emet far apart? Because falsehood is found everywhere, and truth rarely. Why do the letters of sheker each stand on a single foot, and those of emet on a base as wide as a brick? Because truth stands, and falsehood does not stand.
The Alphabet of Rabbi Akiva gives two texts. In the account of the letters presenting themselves before the Holy One, blessed be He, before creation, shin pleads: through me Your name is called, as it is written "this is My name forever," and I am the beginning of the name Shaddai. The answer is no: vanity and falsehood, shav and sheker, are called through you; falsehood has no feet, and neither do you; how could the world be created through a letter that has no feet? And shin went out, downcast. In the other version, shin evokes the teeth, shinayim, of the wicked, which the Holy One, blessed be He, will break three times, in this world, in the days of the Messiah and in the World to Come, "just as this shin has three branches."
The sources make shin a letter of fire and of sharpness. It opens the name Shaddai and it opens the word falsehood; the Talmud and Rabbi Akiva both say that falsehood cannot stand. And Sefer Yetzirah recalls that the fire of shin never goes alone: air decides between it and the water of mem, like the pointer of a scale.