The dimensions of a matrix are the number of rows (horizontal numbers) and columns (vertical numbers); it is written as the rows first, columns second. The following are some simple examples of matrices—all differing in dimensions:

A 3 by 2 (3 × 2) matrix:

A 2 by 3 (2 × 3) matrix:

A 4 by 4 (4 × 4) matrix (when the row and column dimensions are the same, it is called a “square matrix”):