Triangular matrix : A matrix is said to be a triangular matrix if, either 1.every element above principal diagonal is zero. 2.1.every element below principal diagonal is zero.  upper triangular matrix : A matrix is said to be upper triangular if, every element below principal diagonal is zero. Lower triangular matrix : matrix is said to be a triangular matrix if, every element above principal diagonal is zero. Note ( both of eg. listed above) transpose of matrix : if A is a matrix of type m×n obtained by interchanging it's rows and column is called tranpose of A and is denoted by A' or A^T.