What's the opposite of "main diagonal"?

Question

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?

Answer 1

They are known as off-diagonal elements.

Check out the Wikipedia links:

• https://en.wikipedia.org/wiki/Main_diagonal
• https://en.wikipedia.org/wiki/Off-diagonal_element