Possession of a dorsal, hollow nerve cord.

The following are all four defining chordate characteristics:

1. Notochord (chordate characteristic "1")
2. Dorsal, hollow (chordate characteristic "2")
3. Pharyngeal clefts or pharyngeal slits (chordate characteristic "3")
4. Post-anal muscular tail (chordate characteristic "4")

See also phylum Chordata.