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 . 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 square matrix, , is the collection of entries such that . That is, it runs from the top right corner to the bottom left corner:

• Diagonal matrix
• Trace
• Anti-diagonal matrix