tav

The 22nd and last letter of the Hebrew alphabet is tav (or taw). The Hebrew word tav תו means "mark, sign" and this was the early shape of the letter as well. David Sacks writes:

The word taw meant "mark", probably as in an agricultural identifying sign, like a cattle brand or the dye mark that modern Near Eastern shepherds daub on their sheep.

In Modern Hebrew tav can mean a musical note or a computer character. From the word tav we get the verb תוה - "to make marks, sketch, outline" - and this is the origin of the phrase tvay hagader תוי הגדר - "the route of the fence."

We have already noted that tav can alternate with the letters dalet, tet and shin. Additionally, Klein points out that tav as a prefix creates nouns and in doing so interchanges often with the letter mem: תרבית / מרבית , תוצא / מוצא.