Keilim, Chapter 17, Mishnah 10. The Mishnah has been occupied with the standard measurements by which the halachah works, and the amah has just been discussed: the ordinary amah is the middle-sized one, made up of twenty-four etzba'ot, six tefachim. Yet the Mishnah also taught that not every amah in the world was that size. There were larger amot in use in the Beit Hamikdash for particular purposes. Our mishnah now turns to the other direction, and introduces us to a smaller amah, one of only five tefachim, which was also employed in the Beit Hamikdash.
Rabbi Meir speaks first, and he lays down a general assumption: "kol ha'amot hayu benoniyot" - all the amot were of the middle size. Whenever a figure in amot appears, whether in a pasuk or in the words of the Chachamim, concerning something in the Beit Hamikdash, we take it to mean the familiar amah of six tefachim. That is where one begins. Rabbi Meir then proceeds to name the cases in which this assumption does not hold.
The Golden Mizbe'ach
His first exclusion is worded "chutz mimizbach hazahav": the golden mizbe'ach is removed from the general rule. This was the mizbe'ach that stood within the Heichal and upon which the ketoret was offered. Its size is recorded in the Torah as an amah in length and an amah in width, and Rabbi Meir understands those amot to be the shorter measure of five tefachim rather than the usual six.
The Keren, the Sovev and the Yesod
Rabbi Meir continues: "veha'keren veha'sovev veha'yesod." These three all belong to the outer mizbe'ach, the large mizbe'ach that stood in the courtyard, where the blood of the korbanot was applied and where the parts designated for burning were consumed on the fire.
- Ha'keren: the four raised projections that rose at the corners of the great mizbe'ach outside. Each of them took up a square measuring an amah by an amah, and Rabbi Meir reckons that amah at five tefachim.
- Ha'sovev: the ledge that encircled that mizbe'ach on every side, an amah in width, set at a height of six amot above the floor of the courtyard. The amah of that width is likewise the short one.
- Ha'yesod: the foundation ledge that ran around the bottom, rising to a height of one amah. Here again the amah being spoken of is the amah of five tefachim.
Rabbi Yehudah's Division
Rabbi Yehudah sets the dividing line somewhere else entirely. For construction, he says, the measure is "amat habinyan shishah tefachim", six tefachim, while for vessels, "veshel keilim chamishah", five. His distinction is not drawn between one portion of the mizbe'ach and another, but between what was built and what was used. Figures given for the structures, the walls of the Beit Hamikdash and the outer mizbe'ach itself, which carries the status of a building, are counted in the ordinary amah of six tefachim. But any amah stated with respect to one of the keilim of the Beit Hamikdash is the shorter amah of five.
Rabbi Yehudah's position thus produces one clean rule: the vessels of the Beit Hamikdash were reckoned by the amah of five tefachim, and everything raised in stone and mortar by the amah of six. Two Tanna'im, two different ways of charting where the full amah yields to the reduced one, and one shared lesson emerging from both: an amah in the Beit Hamikdash need not be the amah of everyday commerce, and the true dimensions of a thing remain unknown until one determines which amah the Torah had in view.