In linear algebra, the main diagonal (sometimes leading diagonal) of a square matrix is the diagonal which runs from the top left corner to the bottom right corner. For example, the following matrix has 1s down its main diagonal:
- <math>\begin{bmatrix}
1 & 0 & 0\\ 0 & 1 & 0\\ 0 & 0 & 1\end{bmatrix}</math> A square matrix like the above in which the entries outside the main diagonal are all zero is called a diagonal matrix. The sum of the entries on the main diagonal of a matrix is known as the trace of that matrix. The other diagonal is called antidiagonal, counterdiagonal or minor diagonal.
