k-t-v — to write
כ.ת.ב (katav) is the writing root: script, letter, address, marriage contract and Scripture itself all come out of these three letters.
The word family
Every word below is built on כ.ת.ב. Same three letters, different vowels and prefixes.
Same three letters as the verb, different vowels. כְּתַב יָד (ktav yad) is handwriting; כְּתַב רַשִׁ״י is Rashi script.
The place you write to. Also the word for an archaeological inscription.
כְּתוּבִים (Ketuvim), "the Writings", is the third division of the Hebrew Bible — the K in Tanakh.
The Hitpa'el here is reciprocal — two people writing at each other.
What I tell students about this root
This is the root to learn first if you want to feel how the system works, because it is regular in every binyan and the meanings stay honest. Write, dictate (cause to write), correspond (write to each other), be written. Once כ.ת.ב clicks, the same shapes carry over to hundreds of verbs you have not met yet.