Sentence:  The protruding basal oxygens are themselves displaced slightly in the x-direction so that the shift required is either 0.4 + δÅ (overshift) or 0.4 -δ Å (<WSD>undershift</WSD>). 
Meaning of the word marked with <WSD> tags:  (mineralogy) The amount of displacement in the layers in a crystal lattice that places the layers too close together.