The first occurrence.
And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. Gen 1:6
The primitive root.
Strong’s H8432 tavek, a concrete noun meaning, “midst, middle, among;” from an unnamed, unused root meaning, “to sever.” The 3-letter root of the noun is tav + vav + kaph.
tav ת = crossed sticks, thus mark, sign, signal, monument
vav ו = the tent peg, thus add, secure, hook
kaph כ, ך = the open palm, thus bend, open, allow, tame
The story: Marks (tav) or lines are added (vav) to the open palm of the hand (kaph) when it is bent in the middle.