Then represents the conjunction waw (“and”), but it may be omitted. You shall make a mercy seat is literally “you [singular] shall make a kapporeth.” The word probably comes from the verb kipper, which means “to cover over” in the sense of covering over, or atoning for, sins. So it really has the double meaning of “cover” (New Jewish Publication Society’s Tanakh) and “atonement.” (See the comment on “atonement” at 29.33.) New International Version calls it “an atonement cover,” and Fox uses the term “purgation-cover.” Others prefer to call it a “propitiatory,” meaning a place for making atonement. In verse 22 (see also Lev 16.2) we are told that the space just above the kapporeth and between the cherubim is where the LORD will appear to Moses or to the high priest. Here, however, it is probably best to translate it simply as the “cover,” or “lid,” for the “Covenant Box,” but a footnote explaining this double meaning may be helpful to the reader. (Mercy seat, the traditional English rendering, probably originated from Martin Luther‘s German translation of kapporeth as Gnadenstuhl.)

Of pure gold suggests not only that was it “clean” but also that it was heavy. Two cubits and a half shall be its length means that it was “45 inches long” (Good News Translation), or “110 centimeters.” And a cubit and a half its breadth means that it was “27 inches wide,” or “66 centimeters.” Note that this is the exact size of the “Covenant Box.” The thickness of the cover, however, is not indicated.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .