Main diagonal

In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, or major diagonal) of a matrix ${\displaystyle A}$ is the collection of entries ${\displaystyle A_{i,j}}$ where ${\displaystyle i=j}$. All off-diagonal elements are zero in a diagonal matrix. The following three matrices have their main diagonals indicated by red 1's:

The antidiagonal (sometimes counterdiagonal, secondary diagonal, trailing diagonal or minor diagonal) of a dimension ${\displaystyle N}$ square matrix, ${\displaystyle B}$, is the collection of entries ${\displaystyle B_{i,j}}$ such that ${\displaystyle i+j=N+1}$. That is, it runs from the top right corner to the bottom left corner:

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