A drag chain or pull chain is a chain shift in which the phoneme at the "leading" edge of the chain changes first. In the example above, the chain shift would be a pull chain if changed to first, opening up a space at the position of , which then moved to fill. A push chain is a chain shift in which the phoneme at the "end" of the chain moves first: in this example, if moved toward , a "crowding" effect would be created and would thus move toward , and so forth. It is not known which phonemes changed first during the Great Vowel Shift; many scholars believe the high vowels such as started the shift, but some suggest that the low vowels, such as , may have shifted first.