The first tet to appear in the Torah is in the word tov, good. (Genesis 1:12).
During the six-day creation, after completing a day's work God declared a judgement upon it: "God saw it was good" (Genesis 1:12).
The first two Tables of Stones (Exodus 20:2-14) are a different version from the second one (Deutoronomy 5:6-18).
In the first version all the letters of the alphabet appear, except for tet.
In the second version tet appears in the fifth commandament ulemaan ytav lecha, and you shall have goodness.
The wise explain: God knew that Moses would break the first Tables. If they have contained the letter tet, which represent goodness, that would have meant that all the good of the earth would have been broken. To spare men from this concern, God took the first tet from the first version. (source: Baba Kamma 55a)
In gematria, the number 15 is written with tet and vav, (9+6) to avoid the normal construction yod and he (10+5) which spells a name of God.
Similarly, 16 is written with tet and zayn (9+7) instead of yod and vav (10+6) to avoid spelling part of the Tetragrammaton.
Below you can see the movie where I make a big hebrew letter in clay, letter that is the prototype used for the design of my jewels.