Charles Hockett proposed 13 design features for human language. The idea here is that other communication systems include some of these features, but that only human communication includes the three most advanced features

Humans have this, but so do frogs and coyotes.

Pine beetles have this when they broadcast chemicals.

Sound is guaranteed to fade rapidly

Coyotes can either howl or listen to howls.

Coyotes can hear themselves howl, but deaf children cannot.

Speech does little in terms of helping us eat. Its only use is for talking.

Our words actually refer to things, not just emotions or states.

"Whale" is a small word that refers to a big object. But maybe this follows from #9 and #13.

Words are made up of discrete units.

This has to follow as a result of #11. For a fuller analysis.

And this has to follow as a result of #13

Songbirds also depend somewhat on learning the local dialect.

Hockett thinks this is the crucial feature. Duality of patterning involves the ability of humans to combine patterns on two different levels. On the first level, they put together different sounds or phonemes in a specified order to make a single identifiable unit known as a word or morpheme. If one does not maintain the correct order of sounds in a word, the meaning may be completely different as is the case with the words cat, tack, and act. . On the second level, the words are combined into sentences. If we did not have duality of patterning, the number of expressions we could produce would be quite limited. Each word would have to be a single sound unit. We might be able to produce perhaps 100 such units, but then the capacity of our vocal apparatus to create new items would be used up. Duality allows us to continue to create new words as needed. Because we can produce so many words, duality provides a springboard for the other design feature of productivity and displacement.