It is a separate letter, because it is absent from the Ten Commandments and the names of the ten Sefirot. Teth symbolizes the change of state is the only letter opened up. Teth expresses the safety and refuge, introspection and spiritual quest. But the term most often associated with teth is good, for the first time this letter appears in the Torah is to introduce the word tov, "And Gd saw that it was good" (Genesis 1:4 ).
The layout of this letter (Aramaic) corresponding to a shield. This is also consistent with the snake and snake skin which covered with the shield.
Cover, cover, place, preserve, protection, strength, wall, back, roof. Some believe that the word means teth serpent, this because of its shape.