Transpose
[m]square = {{1, 2, 3}, {4, 5, 6}}; Transpose[square]
MatrixForm[%]
matrix = {{1, 2}, {3, 4}, {5, 6}}; MatrixForm[Transpose[matrix]]
Transpose is its own inverse. Transposing a matrix twice will give you back the same thing you started out with:
Transpose[Transpose[matrix]] == matrix