The most stringent and most helpful rule is the third one, which demands the same symmetry of orbitals around the bond axis, or the line connecting atoms. The logic behind this is shown above. Two atomic s orbitals have total symmetry about the bond axis, and can be combined into a s- MO as shown at (a). Two p orbitals oriented as in (b) have what is termed p (pi) symmetry about the bond axis. If you rotate them 180' around this axis, the electron density will look the same, but the signs of the wave functions in all of the density lobes will be reversed.

Each atomic p orbital has one positive lobe and one negative lobe, and there is a surface plane halfway between on which the electron probability falls to zero. Such a zero- probability surface in a wave function is called a node. The resulting p MO also has a positive lobe, a negative lobe, and a zero-probability node halfway between.