The column rank of a matrix is the dimension of the space spanned by its columns; the row rank of a matrix is the dimension of the space spanned by its rows. Row and column ranks are always equal, so that the term "rank" is unambiguous. A non-square matrix is said to be of full rank if it has the maximal possible rank, that is, the lesser of its dimensions. To be invertible (or non-singular), a square matrix must be of full rank.