Main diagonal
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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:

A matrix like the above in which only the elements of the main diagonal are non-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 original article can be found at: http://en.wikipedia.org/wiki/Main_diagonal.
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