An octave displacement is a register switch in which a melodic line is continued an octave higher or lower than expected after a note on a strong beat, often because of a tessitura issue. Example a illustrates how two octave displacements in the upper part of bar 2 ensure that the first and last pitch are identical, while example b shows where the upper part would end up without these octave displacements:
As is the case with the leap of an octave, an octave displacement is also best continued in the opposite direction.