Multicategoriality
Another factor which creates problems when we try to theorize about grammatical categories is that in some languages, including English, there are many words which are both nouns and verbs:

Even words which are typically members of just one category can be used in the other. (12) (a–b) show verbs appearing as nouns:

These possibilities are not limited to nouns and verbs. Contexts like (14a–b) allow the titles sir and madam, which are certainly not verbs, to be inserted into verb slots:

Badge of honour is typically a noun; but when it takes on the grammatical position associated with adjectives, it assumes a qualifying role. Examples like these are widespread, and they suggest that there is a high degree of fluidity to the English part-of-speech system.
QUESTION Traditionally the first noun in cases like (15) is labelled ‘a noun used as an adjective’. What are the advantages and problems with this? Think of other noun–noun combinations, such as snail mail, week end warrior, etc.
Some languages have even more far-reaching possibilities of multicategoriality. In some languages of the Pacific Northwest of America and Canada, there is a widespread cross-over between noun and verb categories. The following examples are from St’át’imcets (Salishan; British Columbia; Davis and Matthewson 1999: 38):

In (16a) we find the root nk’yáp ‘coyote’ performing a function typically associated with nouns – reference – and supplied with a determiner. The root t’ak ‘go along’ functions as a verb. In (16b) these functions are completely reversed. Now it is nk’yap ‘coyote’ which has the verbal function, and t’ák ‘go along’ the nominal morphology. This possibility for roots to appear in either nominal or verbal contexts is widely found in the Pacific North West of North America, such as Nuu-chah-nulth (Wakashan; British Columbia; often referred to as ‘Nootka’), as well as in other languages of the world, including Tagalog (Austronesian; Philippines): (17) gives some Tagalog examples (Evans and Osada 2005: 368):

The predicate in Tagalog is signalled by clause-initial position; (17) gives a sense of the freedom with which roots can be inserted into this position.
Sign languages also often have words which are able to function equally as nouns or verbs. In DGS (Deutsche Gebärdensprache, German Sign Language), for instance, both the signs in (18) can be translated as both nouns and verbs:

(The handsigns are represented here by their approximate German translation equivalents, given in capital letters.)
Facts like these give us a different way of thinking about the nature of verbhood and nounhood. We began this section by saying that noun hood and verbhood are usually thought of as inherent properties of lexical roots: a word ‘brings’ its status as a noun or verb with it to the sentence. The thoroughgoing multicategoriality found in languages like St’át’imcets, Tagalog and Nuu-chah-nulth, as well as the more limited, but still frequent multicategoriality of a language like English, give us good reason not to talk in this way. Instead, we can think of nouns and verbs as ‘slots’ or contexts available in each clause, each of which comes associated with typical grammatical machinery (TMA markers for verbs/events; plu rality/definiteness etc. markers for nouns). In English, these slots are the phrase-level categories of NP and VP, each of which prototypically has its distinctive grammatical markers. In other languages, the marking of the slot may be purely morphological. But in both cases we can see the gram matical slots themselves as the carriers of the nounhood or verbhood which the word ends up acquiring. These slots display the typical struc ture of a prototype category: prototypically, a noun slot is filled with words denoting perceptible, time-stable referents, and a verb slot is filled with words denoting concrete actions. In some languages, there are roots which are uniquely licensed to appear in either noun or verb slots. In oth ers, a given root may appear in both. But in both cases, the categoriality comes from the slot in the clause, not from the root itself.