Sentence:  The protruding basal oxygens are themselves displaced slightly in the x-direction so that the shift required is either 0.4 + δÅ (<WSD>overshift</WSD>) or 0.4 -δ Å (undershift). 
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 far apart for molecular bonding.