Titzaveh, Shemos 30:2. One by One, Square.

Describing the Mizbei'ach Hazahav,

א ועשית מזבח מקטר קטרת עצי שטים תעשה אתו.  ב אמה ארכו ואמה רחבו רבוע יהיה ואמתים קמתו ממנו קרנתיו.

"an amah in length, and an amah width, it shall be squared, and two amos its height...."
Shai LaTorah II asks that once we have been told that the top is one amah by one amah, to be told that it should be square seems unnecessary and repetitive.  It is redundant.

I asked this question at my kiddush, and got two answers.  One was that in Kodshim, there is a rule of שינה עליו הכתוב לעכב, that if a din is stated once, it is a mitzva, but if one fails to fulfill the instruction, the korban or utensil remains kosher bedi'eved.  Only if the mitzvah or instruction is repeated does it become essential to the kashrus of the avodah or keli.  By saying Rav'uah, the Torah has invoked the rule of שינה עליו הכתוב לעכב.  This sounds like a good teretz, but the Shai LaTorah brings an answer from Reb Chaim that evidently assumes that this answer is not correct.

The answer he brings from Reb Chaim is that from here we see that the din ravu'a by the mizbei'ach is not merely a result of being one amah by one amah, but an independent requirement.  His words-

על כרחך היוצא מזה דהא דהמזבח "רבוע" הוא, אינו רק תוצאה ל"אמה ארכו ואמה רחבו," אלא דהוא דין מיוחד שעל המזבח להיות רבוע.

I would explain Reb Chaim as saying that even if the dimensions of the mizbei'ach didn't matter, it would have to be ravu'ah.  It so happens, that besides the din of ravu'ah, the dimensions do matter, and they happen to be one long and one wide.  But the essential requirement of ravu'ah is independent of the din of the dimensions.

My wife answered the question by saying that a rhombus can also be described as having sides of one amah by one amah, so of course the Torah needed to tell you that the mizbei'ach had to be square.  The idea is that you can describe something as being one amah long and one amah wide even if the corners are not exactly ninety degrees.

Since she never reads this website, I can admit that she is making a good point.

Rabbi Dr. GS independently said that one by one might also describe a cylinder, but I disagree.  The only place in Tanach that I can remember where the dimensions of a cylinder are described is the Yam shel Shlomo, and the passuk there just mentions the diameter and the circumference- Divrei HaYamim II 4:2,

 ויעש את הים, מוצק  עשר באמה משפתו אל שפתו עגול סביב וחמש באמה קומתו וקו שלשים באמה יסב אתו סביב 

The Rabbi Dr. wrote back:

Could respond where it must be cylindrical it describes it in terms as per Yam shel Shlomo and if it must be square it says merubah

If left ambiguous then either shape would be acceptable 

(By the way, the previous passuk describes the Mizbei'ach Ha'Olah:

ויעש מזבח נחשת עשרים אמה ארכו ועשרים אמה רחבו ועשר אמות קומתו.  )

Another thing I was wondering about is why Reb Chaim didn't ask the question by the Choshen, which is described in 28:16 as being square, a zeres by a zeres- רבוע יהיה כפול זרת ארכו וזרת רחבו.