ammah

A cubit is the basic measuring stick of the Hebrew Bible, the way a building, a boat, a piece of furniture, or a person's height gets sized. It is named for the forearm, because that is what it began as: the length from the elbow to the fingertip. In round modern terms it runs about eighteen inches, though no two arms are identical, which is exactly why the Bible sometimes pins the unit down more precisely.

Most occurrences are plain measurements, and they cluster where the text is describing something built to specification: Noah's ark, the tabernacle and its furnishings, Solomon's temple, and above all Ezekiel's visionary temple, which is paced out cubit by cubit. The word is overwhelmingly a builder's and surveyor's term, not a poetic one.

The two senses below are not different units but the same cubit named at two standards. Usually the text simply assumes the ordinary forearm length everyone knew. A few times, especially for sacred architecture, it specifies a longer official cubit, the common one plus the width of a hand. The qualifiers tell the reader which standard is in view, much as we might distinguish a statute mile from a nautical one.

In one passage the word does not measure anything at all. When Isaiah sees the temple in his vision, the sound of the seraphim's cry makes the lower structure of the doorway shake. Here the word names the solid base of the door-frame, the sockets or foundations that anchor the threshold and posts.

Most scholars treat this as a separate word that happens to be spelled the same as the word for cubit, rather than a stretched use of the measurement term. A unit of length cannot tremble; a building's footing can. The image is of the temple itself trembling at the holiness of God, shaken from its very base.