Prepositions are 'small words' used to describe the relationship between other entities indicated by other words in a sentence. Well-known by their linking functions, prepositions are regarded as rather meaningless unless in context. Generally, prepositions often denote spatial or temporal relations between their complement and the element they modify, although they can also denote relations regarding means, manner, instrument, cause, purpose, and so on. In theory of semantic change, prepositions are often considered as the results of the grammaticalisation from nouns or verbs. After grammaticalisation, prepositions are often considered as having grammatical meanings rather than the lexical meanings of their nominal or verbal origins.