Tav is the last letter of the hebrew alphabet.
Pronounciation: T or TH
Example: torah, Torah.
In gematria, tav represents the number 400.
The letter tav represents truth and perfection.
Tav is believed to be derived from the egyptian hieroglyph meaning "mark".
Tav is the last letter of the Hebrew word emet, which means 'truth'. The midrash explains that emet is made up of the first, middle, and last letters of the hebrew alphabet (alef, mem, and tav).
Sheqer (falsehood), on the other hand, is made up of the 19th, 20th, and 21st (and penultimate) letters.
Thus, truth is all-encompassing, while falsehood is narrow and deceiving.
In gematria, tav represents the number 400, the largest single number that can be represented without using the sophit (final) forms (see kaf, mem, nun, pe and tzadi).
The letters shin and tav are next to each other in the alphabet, regardless of fact that they express opposite concepts: shin stands for sheqer, false and tav stands for emet, truth (source: Shabbat 105)
Contrary to most cases where the first letter of the word is used to give meaning, in the case of emet, truth the last letter tav is used to give meaning. This suggests at the nature of truth: even if in the beginning truth seems less attractive than falsehood, in the end it (truth) prevails.
Truth is eternal, but when the alef which is the smallest value of emet, truth is removed from it, then met, death remains. (source: Maharal)
In jewish mithology it was the word emet that was carved into the head of the golem which ultimately gave it life. But when the letter aleph was erased from the golem's forehead, what was left was met,dead. And so the golem died.
The tav shows us that the end was set from the beginning, as tav is the final letter of beresheet, “In the beginning”, the first word of the Torah. It is the idea that the Creator set in motion all of existence in order to reach a final state of perfection, the fulfillment of all of creation.
"From aleph to tav" describes something from beginning to end, the hebrew equivalent of the english "From A to Z."
The tav shows us that the end was set from the beginning, as tav is the final letter of beresheet, in the beginning, the first word of the Torah. It is the idea that the Creator set in motion all of existence in order to reach a final state of perfection, the fulfillment of all of creation.
Upper you can see a movie showing the production of a big clay prototype of the hebrew letter aleph. The prototype later on will be used to design a jewel.