MAIN DIAGONAL

In linear algebra, the 'main 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:
:
1 & 0 & 0\
0 & 1 & 0\
0 & 0 & 1end{bmatrix}
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'.