• An identity matrix is a square matrix with ones on the main diagonal (upper left to lower right) and zeroes off the diagonal. This matrix is usually given the symbol $I$.
$$\begin{pmatrix}0&0\\0&0\\0&0 \end{pmatrix},\quad \begin{pmatrix}1&0&0\\0&1&0\\0&0&1\end{pmatrix}.$$