Iconicity is a relationship of resemblance or similarity between the two aspects of a sign: its form and its meaning. An iconic sign is one whose form resembles its meaning in some way. The opposite of iconicity is arbitrariness. In an arbitrary sign, the association between form and meaning is based solely on convention; there is nothing in the form of the sign that resembles aspects of its meaning. The Hindu-Arabic numerals 1, 2, 3 are arbitrary, because their current form does not correlate to any aspect of their meaning. In contrast, the Roman numerals I, II, III are iconic, because the number of occurrences of the sign I correlates with the quantity that the numerals represent. Because iconicity has to do with the properties of signs in general and not only those of linguistic signs, it plays an important role in the field of semiotics—the study of signs and signaling. However, language is the most pervasive symbolic communicative system used by humans, and the notion of iconicity plays an important role in characterizing the linguistic sign and linguistic systems. Iconicity is also central to the study of literary uses of language, such as prose and poetry. There are various types of iconicity: the form of a sign may resemble aspects of its meaning in several ways: it may create a mental image of the concept (imagic iconicity), or its structure and the arrangement of its elements may resemble the structural relationship between components of the concept represented (diagrammatic iconicity). An example of the first type is the word cuckoo, whose sounds resemble the call of the bird, or a sign such as RABBIT in Israeli Sign Language, whose form—the hands representing the rabbit's long ears—resembles a visual property of that animal. An example of diagrammatic iconicity is vēnī, vīdī, vīcī, where the order of clauses in a discourse is understood as reflecting the sequence of events in the world. Iconicity is found on all linguistic levels: phonology, morphology, syntax, semantics, and discourse. It is found both in spoken languages and in sign languages. However, sign languages, because of the visual-gestural modality through which they are transmitted, are much richer in iconic devices, and therefore offer a rich array of topics and perspectives for investigating iconicity, and the interaction between iconicity and language structure.