Human language is unique in comparison to other forms of communication, such as those used by non-human
animals. Communication systems used by other animals such as
bees or non-human
apes are closed systems that consist of a closed number of possible things that can be expressed.
In contrast, human language is open-ended and
productive, meaning that it allows humans to produce an infinite set 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, where a finite number of meaningless elements (e.g. sounds, letters or gestures) can be combined to form units of meaning (words and sentences). Furthermore, the symbols and grammatical rules of any particular language are largely arbitrary, meaning that the system can only be acquired through social interaction. The known systems of communication used by animals, on the other hand, can only express a finite number of utterances that are mostly genetically transmitted.