The orientation of an atomic orbital is governed by

Quantum numbers may be defined as a set of four numbers which give complete information about the electron in an atom, i.e., energy, orbital occupied, size, shape and orientation of that orbital and the direction of electron spin. Magnetic quantum number is required to explain the fact that when the source giving the line spectrum is placed in a magnetic field, each spectral line splits in to a number of lines. The magnetic quantum number tells the number of orientations which the orbitals present within the same sub-shell can take up.