In contrast, human language is open-ended and productive , meaning that it allows humans to produce a vast range of utterances from a finite set of elements, and to create new words and sentences. This is possible because human language is based on a dual code, in which a finite number of elements which are meaningless in themselves (. sounds, letters or gestures) can be combined to form an infinite number of larger units of meaning (words and sentences).  However, one study has demonstrated that an Australian bird, the chestnut-crowned babbler, is capable of using the same acoustic elements in different arrangements to create two functionally distinct vocalizations.  Additionally, pied babblers have demonstrated the ability to generate two functionally distinct vocalisations composed of the same sound type, which can only be distinguished by the number of repeated elements.