A matrix is a rectangular array of numbers. Each entry in the matrix is called an element. Matrices are classified by the number of rows and the number of columns that they have; a matrix A with m rows and n columns is an m × n (said 'm by n') matrix, and this is called the order of A. Example

Given

Then A has order 2×3 (rows first, columns second.) The elements of A can be denoted by a_ij , being the element in the ith row and jth column of A. In the above case, a11 (row 1, column 1) =1, a23 (row 2, column 3) =0, etc.