This will rotate a specified face (hkl) to lie flat in the screen or paper (face-normal parallel to x observer), or a vector (uvw) to be perpendicular to the screen or paper. To align a face, it is also necessary to specify a vector lying in the face which will be vertical in the screen or paper (z direction). Conversely, to align a vector, it is necessary to specify a face, containing the vector, whose normal will be vertical. If a vector [uvw] lies in a face (hkl), the indices satisfy the condition hu + kv + lw = 0. If clinographic viewing is in effect, it is turned off when the face is aligned, to simplify any subsequent rotations.
In the Cartesian mode, faces and vectors are equivalent - that is, to align a face is also to align a vector with the same indices.
You can elect to use the default alignment vector or face, instead of specifying it. The default vector will be the intersection of the face (hkl) with the face (100), whose indices are given by the cross product (hkl)x(100). If the face (hkl) is (100), the default alignment vector will be the vector [001].