In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, or major diagonal) of a matrix is the collection of entries where . 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 Harrison diagonal, secondary diagonal, trailing diagonal, minor diagonal, or bad diagonal) of a dimension square matrix, , is the collection of entries such that for all . That is, it runs from the top right corner to the bottom left corner.