Tav, the final letter of the Hebrew alphabet. Tav is the sign or seal of a covenant. It was often used as a mark in the same sense as our signature is today, especially of those who could not read or write. Conceptually, tav is a wounding, as the sealing of a covenant required the wounding of the sacrifice, whose blood sealed the covenant. It resembles a doorpost, as the blood was applied on in Mitzraim (Egypt), thus sealing that household and sparing them from the tragedy of the firstborn.
But you, dear friends, build yourselves up in your most holy faith, and pray in union with the Ruach HaKodesh. Thus keep yourselves in God’s love, as you wait for our Lord Yeshua the Messiah to give you the mercy that leads to eternal life. -- Jude:1:20-21