Human language is organized at two levels or layers simultaneously. This property is called duality (or “double articulation”). In speech production, we have a physical level at which we can produce individual sounds, like n, b and i. As individual sounds, none of these discrete forms has any intrinsic meaning. In a particular combination such as bin, we have another level producing a meaning that is different from the meaning of the combination in nib. So, at one level, we have distinct sounds, and, at another level, we have distinct meanings. This duality of levels is, in fact, one of the most economical features of human language because, with a limited set of discrete sounds, we are capable of producing a very large number of sound combinations (e.g. words) which are distinct in meaning.

 Among other creatures, each communicative signal appears to be a single fixed form that cannot be broken down into separate parts. Although your dog may be able to produce woof (“I’m happy to see you”), it does not seem to do so on the basis of a distinct level of production combining the separate elements of w + oo + f. If the dog was operating with the double level (i.e. duality), then we might expect to hear different combinations with different meanings, such as oowf (“I’m hungry”) and foow (“I’m really bored”).