In linear algebra, the **main diagonal** (sometimes **leading diagonal** or **major diagonal** or **primary diagonal** or **principal diagonal**) of a matrix is the collection of entries where is equal to .

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 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 square matrix is known as the trace of that matrix.

The main diagonal of a rectangular matrix is the diagonal which runs from the top left corner and steps down and right, until the right edge or the bottom edge is reached.

The diagonal of a square matrix from the top right to the bottom left corner is called **antidiagonal**, **counterdiagonal**, **secondary diagonal**, or **minor diagonal**.

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