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In linear algebra, the **main diagonal** (sometimes **principal diagonal**, **primary diagonal**, **leading diagonal**, **major diagonal**, or **good diagonal**) of a matrix is the list of entries where . All off-diagonal elements are zero in a diagonal matrix. The following four matrices have their main diagonals indicated by red ones:

See also: Anti-diagonal matrix |

The **antidiagonal** (sometimes **counter diagonal**, **secondary diagonal**, **trailing diagonal**, **minor diagonal**, **off diagonal**, or **bad diagonal**) of an order 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.