# What's the opposite of "main diagonal"?

This matrix has only '1's on its main diagonal:

$A = \begin{pmatrix} 1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 1 & 0\\ 0 & 0 & 0 & 1 \end{pmatrix}$

But whats the opposite of main diagonal?

I want to say something like: "There are only '0's on the NOT-main-diagonal"

Can someone help me with a word?

• "not-main-diagonal" can mean either "not-(main-diagonal)" (i.e. the entries which are not on the main diagonal), or "(not-main)-diagonal" (i.e. the diagonal which goes from top right to bottom left, and therefore is not the main diagonal). I think you mean the first one, and the answer below is correct in that case. But just to be sure: which one do you mean? May 27, 2018 at 7:51
• yes, the first. May 27, 2018 at 7:57