Here's why:

* Complexity: Chordates are complex organisms with specialized tissues, organs, and organ systems. This level of organization is only possible in multicellular organisms.

* Defining Characteristics: The defining characteristics of chordates (notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail) are all structures that are too complex to exist within a single cell.

* Examples: Humans, fish, birds, reptiles, amphibians - all of these are chordates and are clearly multicellular.

Let me know if you have any other questions about chordates!