Parts of speech refer to the classification of different categories of lexical items based on their syntactic or morphological behavior. The two most studied parts of speech are nouns and verbs.
In the lexicon, we can distinguish between functional words, which form a closed class, and lexical/content words, which form an open class. Nouns, verbs, adjectives, and adverbials are lexical words, while pronouns, adpositions, conjunctions, numerals, quantifiers, and interjections are functional words.
In LIS, and many other sign languages as well, it is not always easy to identify different parts of speech, and determining the part of speech that a given sign belongs to can be difficult too. For example, many verbs have a nominal counterpart with the same (or a very similar) phonological form: in these cases, distinguishing the verb and the noun that are semantically related can be challenging.
Another difficulty is the fact that we can find non-manual realisations for certain categories of parts of speech. For example, some adjectives can be expressed with a manual form, but may also be realised non-manually when modifying a noun.
Moreover, there are elements listed as a category of parts of speech that in sign languages may have no manual realisation at all. This is, for example, the case of adpositions. Although in some cases they can be expressed by a manual sign, very frequently the information conveyed by an independent adposition is expressed by means of relative locations in the signing space.
To determine the class a sign belongs to, it is therefore necessary to consider different aspects, like its position in the sentence, the non-manual markers that accompany its production and if it agrees with other elements.
In LIS, we can identify the following categories: nouns, verbs, determiners, adjectives, pronouns, adverbs, adpositions, and conjunctions.