All vertebrates are chordates but all chordates are not vertebrates.

Explanation

(i) In some chordates, the notochord is present only in the tail region of larvae.
(ii) In some other chordates, notochord is present throughout the body length.
(iii) In some chordates, the notochord is replaced by vertebral column.
(iv) Due to this, all chordates do not get classified in the same group.
(v) Hence, all vertebrates are chordates but all chordates are not vertebrates.