Kaf is the eleventh letter and has the value twenty. Its name is a common word: kaf means the palm of the hand and the pan of a scale, and Sefer Yetzirah speaks of "kaf zekhut and kaf chovah," the pan of merit and the pan of guilt. Kaf has a final form, written at the end of a word; the Talmud teaches that these final forms, Mem, Nun, Tzadi, Peh, Kaf, had been forgotten and that the prophets restored them. Sefer Yetzirah places it in the group Gimel, Yod, Kaf, Kof.
In Sefer Yetzirah, Kaf is one of the seven doubles, with a hard or soft sound. G-d "made the letter Kaf reign and bound a crown to it," and through it formed Chamah, the sun, in the world, the fourth day of the week in time and the left ear in the body. Like each double, it stands between a good and its opposite, which the book lists: life and death, wealth and poverty, dominion and servitude.
In tractate Shabbat, the children's sentence comes to its end: "if you do so, the Holy One, blessed be He, feeds you, is gracious to you, does good to you, gives you an inheritance, and binds for you a crown in the world to come." Kaf gives keter, the crown. The whole sequence, from Zayin to Kaf, flows from "gemol dalim": do good to the poor.
What is this crown of the world to come? Tractate Berakhot reports a saying Rav often had on his lips: "The world to come is not like this world. There is no eating or drinking there, no commerce, no jealousy, no hatred, no rivalry; but the righteous sit with their crowns on their heads and delight in the radiance of the Shekhinah." One may bring this saying together with the crown promised by the children of Shabbat 104a. The Alphabet of Rabbi Akiva sees in Kaf the palm struck against the other, in great joy, at the feast of the righteous in the world to come.
Twenty, the open palm and the crowned head. The word kaf means the palm and the pan in which merit is weighed; the Talmud reads in the letter the crown awaiting the one who gives. One may read these two images together: the palm that opens for the poor, and the crown placed on the head of the righteous.