np.r_[] It concatenates arrays along first axis. np.c_[] It concatenates arrays along second axis. a = np.array([[1, 2, 3], [11,22,33]] ) b = np.array([[4, 5, 6], [44,55,66]] ) np.r_[a,b] >>> array([[ 1, 2, 3], [11, 22, 33], [ 4, 5, 6], [44, 55, 66]]) np.c_[a,b] >>> array([[ 1, 2, 3, 4, 5, 6], [11, 22, 33, 44, 55, 66]])