Again, solid lines locate electron density maxima when the phase is 0° (+|F_hkl|) and dashed lines locate electron density maxima when the phase is 180° (-|F_hkl|). Where three of these maxima intersect, there must be a large amount of electron density, especially if the amplitudes of the three waves are all large (large |F_hkl|). At these triple intersection points, in fact, there must be an atom!

Inspection of this figure indicates that there are only two types of triple intersection points: when all three phases are positive or when two phases are negative and one is positive.
This observation can be summarized as:

where s_hkl represents the sign of |F_hkl| and ~ means “is probably equal to”.

This is a general relationship and can be applied to other reflection triplets: