 The exact details of the shift are unknown, and it may have progressed in a variety of ways before arriving at the final situation.The three stages listed above show the progression of a "pull chain", in which each change leaves a "gap" in the phonological system that "pulls" other phonemes into it to fill the gap.But it is also conceivable that the shift happened as a push chain, where the changes happened in reverse order, with each change "pushing" the next forward to avoid merging the phonemes.