Misconception: The Luchot [Tablets] on which the 10 Commandments were engraved were rounded at the top (as in most illustrations and atop synagogue arks).
Fact: According to tradition, they were rectangular parallelepipeds (cuboids), with no superior arc. They each measured 6 tefachim [handbreadths] high by 6 tefachim wide by 3 tefachim thick.
Background: The Luchot, the Tablets of the Law, as two adjacent rectangles, taller than wide and rounded at the top, is today such an ubiquitous symbol of Judaism, and appears in so many synagogues, that it may be considered the principal symbol of Judaism.  Historically this has not always been the case.
Up until the Middle Ages, Jews did not use the Tablets as a symbol and they rarely depicted them in drawings. When they were shown, such as in several 13th century manuscripts, they were shown within the Ark as two rectangular forms.
The current standard perception of the Tablets derives from Christian art. These works had various portrayals of the Tablets. In the east and in Byzantine art it was represented, in direct contradiction to the Biblical text (Exodus 31:18), as a scroll. In Italy, from ancient times through Romanesque and Gothic periods and up until the 16th century, the Luchot were portrayed as two rectangular tablets. In France, from at least the 12th century, it was the familiar rounded form that at a certain point supplanted all the other forms; so much so that from the 16th century on it is almost impossible to find Tablets represented in any other fashion.
Scholars have traced the origin of this form with relative ease. There are several features that almost always go together that betray its source. When portrayed as rectangular, the Tablets are always separate. When they have a rounded top, they are usually adjacent and even attached. Furthermore, when rounded they usually have a frame. All of these indicate a familiar form — the diptych. The diptych was an ancient writing tablet having two hinged leaves that was used in various forms through the Middle Ages. Representing the Luchot as a diptych lies in its use as a writing tablet, and probably originated in England, where the earliest extant example of rounded Luchot can be found in an 11th century manuscript.
In traditional Jewish sources, the size and shape of the Luchot are derived in the Talmud based on the dimensions of the Ark. This approach led the Talmud (Baba Batra 14a) to conclude that each tablet was 6x6x3 tefachim. In addition, the Talmud declares that after all of the items were placed in the Ark it was totally full, implying that the Tablets were cuboids, not rounded at the top.
Most contemporary synagogues and institutions have no qualms about depicting the Luchot in a format that originated in non-Jewish art. However, it is well known that the recent Lubavitcher Rebbe was adamantly against this and would not let any of his institutions use a fig u re of rounded Luchot on their stationery. Other contemporary authorities are less adamant about this.
Rav Shternbuch (Tshuvot v’Hanhagot 2:535) states clearly that in Jewish tradition the Luchot were not rounded. But those used to decorate synagogue arks are not of the same size as the original either. They are not meant to be replicas, but artistic representations or symbols of the Luchot. If asked initially, he would favor using square ones, but there is no harm in having rounded Luchot. He does note that Rav Yaakov Kanievsky, “the Steipler, ” was very particular about this issue and wrote that one should use only square d Luchot.
1. The Tablets were used as a symbol of Judaism by the Christians before the it was used by the Jews. For example, Henry III, who ruled England from 1216 to 1272, issued a decree that all Jews had to wear an identifying badge that was in the shape of two tablets. The Luchot do not appear as a symbol among the Jews until about the 15th century. For sources on this and much of the other material in this background section, as well as many figures depicting the Luchot in various shapes, see: Ruth Mellinkoff, The Round-topped Tablets of the Law: Sacred Symbol and Emblem of Evil, Journal of Jewish Art, 1(1974), pp. 28-43 and Gad B. Sarfatti, “The Tablets of the Law as a Symbol in
Judaism” that appeared in Hebrew (1985) and English (1990) in The Ten Commandments in History and Tradition,
Magnes Press, Jerusalem, edited by Ben-Zion Segal and Gershon Levi, pages 383-418.
2. Although this was the shape of the ancient oriental stele, this is probably not the origin since it is unlikely that Medieval European artists would use ancient Egyptian or Assyrian stelae as their model.
3. In the Mishnah it is referred to as a pinkas with two levachim. See, for example, Shabbat 12:4 and Jastrow on “luach” (page 696) and “pinkas” (with a yud, page 1165).
4. Not 12th century France, as others have proposed.
5. See also Baraita di-Melechet ha-Mishkan, ed. Ish-Shalom, Vienna 1908, p. 40 and Bamidbar Rabbah 4:21. There are also rabbinic passages about the Luchot that are clearly aggadic. For example Breishit Rabbah states that the tablets were 136 fingers by 28 fingers, the “fingers” being the Fingers of God, “whose little Finger is as large as the entire world.”
6. The Yerushalmi (Shekalim 6:1; 16a-b; Sotah 8:3; Taanit 4:5), according to the Gra’s textual emendation, gives the measur ements as 6 tefachim high by 3 (not 6) wide and 3 thick.
7. I have seen a suggestion (by Shlomit Perlman, Zomet Institute Shabbat B’Shabbato) that the rounded tops of the
Luchot may be related to the Midrash, quoted by Rashi, to Exodus 34:1. God instructed Moses to, “Carve for yourself two Tablets.” The Midrash teaches that Moses was permitted to keep the stone shavings and thereby became wealthy. If the tablets were simple rectangles, it is argued, would God have given Moses a stone that was too large? Rather, the leftover stone was the result of rounding the tops of the tablets. I find this explanation puzzling. Why is it difficult to assume that God gave Moses a stone that was too large, but not that He gave him one that was the “wrong” shape?
8. See Sha’arei Halachah U’minhag, OC:1 page 199. It is also found in Sichos in English. This is based on addresses given by the Rebbe on Shabbat Parshat Ki Tissa, 5741 and on Simchat Torah 5742.
Reprinted from JEWISH ACTION Magazine, Summer 5759/1998 issue