Function words (or grammatical words or synsemantic words or structure-class words) are words that have little lexical meaning or have ambiguous meaning but instead serve to express grammatical relationships with other words within a sentence or specify the attitude or mood of the speaker. They signal the structural relationships that words have to one another and are the glue that holds sentences together. Thus they serve as important elements to the structures of sentences.

1 Consider the following sentences (1) and (2):

(1) The winfy prunkilmonger from the glidgement mominkled and brangified all his levensers vederously.

(2) Glop angry investigator larm blonk government harassed gerfritz infuriated sutbor pumrog listeners thoroughly.

In sentence (1) above the content words have been changed into nonsense syllables but it is not difficult for one to posit that winfy is an adjective prunkilmonger glidgement levensers as nouns mominkled brangified as verbs and vederously as an adverb based on clues like the derivational and inflectional morphemes. (The clue is in the suffixes: -y indicates adjectives such as "wintery"; -er -ment and -ers indicates nouns such as "baker" "battlement" and "messengers"; -led and -fied suggests verbs such as "mingled" and "clarified"; and -ly is that of adverbs such as "vigorously"). Hence even without lexical meaning the sentence can be said to be rather "meaningful". However when the reverse is done and the function words are being changed to nonsense syllables as in sentence (2) the result is a totally incomprehensible sentence as the grammatical meaning which is signaled by the structure words is not present. Hence function words provide the grammatical relationships between the open class words and helps create meaning in sentences.

Words that are not function words are called content words (or open class words or lexical words or autosemantic words): these include nouns verbs adjectives and most adverbs although some adverbs are function words (e.g. then and why). Dictionaries define the specific meanings of content words but can only describe the general usages of function words. By contrast grammars describe the use of function words in detail but treat lexical words in general terms only.

Function words might be prepositions pronouns auxiliary verbs conjunctions grammatical articles or particles all of which belong to the group of closed-class words. Interjections are sometimes considered function words but they belong to the group of open-class words. Function words might or might not be inflected or might have affixes.

Function words belong to the closed class of words in grammar in that it is very uncommon to have new function words created in the course of speech whereas in the open class of words (that is nouns verbs adjectives or adverbs) new words may be added readily (such as slang words technical terms and adoptions and adaptations of foreign words). See neologism.

Each function word either gives some grammatical information on other words in a sentence or clause and cannot be isolated from other words or it may indicate the speaker's mental model as to what is being said. Grammatical words as a class can have distinct phonological properties from content words. Grammatical words sometimes do not make full use of all the sounds in a language. For example in some of the Khoisan languages most content words begin with clicks but very few function words do.2 In English only function words begin with voiced th- (see Pronunciation of English th). The following is a list of the kind of words considered to be function words: articles the and a. In some inflected languages the articles may take on the case of the declension of the following noun. pronouns inflected in English as he him she her etc. adpositions uninflected in English conjunctions uninflected in English auxiliary verbs forming part of the conjugation (pattern of the tenses of main verbs) always inflected interjections sometimes called "filled pauses" uninflected particles convey the attitude of the speaker and are uninflected as if then well however thus etc. expletives take the place of sentences among other functions. pro-sentences yes okay etc. See also Look up function word in Wiktionary the free dictionary. Content word Stop word in computer science References Klammer Thomas Muriel R. Schulz and Angela Della Volpe. (2009). Analyzing English Grammar (6th ed).Longman. Westphal E.O.J. (1971) "The click languages of Southern and Eastern Africa" in Sebeok T.A. Current trends in Linguistics Vol. 7: Linguistics in Sub-Saharan Africa Berlin: Mouton  External links List of function words v d eLexicography Types of reference works Dictionary  Glossary  Lexicon  Thesaurus Types of dictionaries Bilingual  Biographical  Conceptual  Defining  Electronic  Encyclopedic  Language for specific purposes dictionary  Machine-readable  Maximizing  Medical  Minimizing  Monolingual learner's  Multi-field  Phonetic  Picture  Reverse  Rhyming  Rime  Single-field  Specialized  Sub-field  Visual Lexicographic projects Lexigraf  WordNet Other List of lexicographers  List of online dictionaries v d eLexicology Major terms Lexicon  Idiolect  Word  Lexis  Lexical unit Elements Morpheme  Grapheme  Glyphs  Phoneme  Sememe  Seme  Lexeme  Lemma  Meronymy  Chereme Semantic relations Holonymy  Hyponymy  Troponymy  Idiom  Synonym  Antonymy  Lexical semantics  Semantic net Fonctions Function word  Headword Fields Morphology  Controlled vocabulary  English lexicology and lexicography  Lexicographic error  Lexicographic information cost  Linguistic prescription  Specialised lexicography  International scientific vocabulary v d eLexical categories and their features Noun Abstract/Concrete  Adjectival  Agent  Animate/Inanimate  Attributive  Collective  Common/Proper  Countable  Deverbal  Initial-stress-derived  Mass  Relational  Strong  Verbal  Weak Verb Verb forms Finite  Non-finite Attributive  Converb  Gerund  Gerundive  Infinitive  Participle (adjectival  adverbial)  Supine  Verbal noun Verb types Accusative  Ambitransitive  Andative/Venitive  Anticausative  Autocausative  Auxiliary  Captative  Catenative  Compound  Copular  Defective  Denominal  Deponent  Ditransitive  Dynamic  ECM  Ergative  Frequentative  Impersonal  Inchoative  Intransitive  Irregular  Lexical  Light  Modal  Monotransitive  Negative  Performative  Phrasal  Predicative  Preterite-present  Reflexive  Regular  Separable  Stative  Stretched  Strong  Transitive  Unaccusative  Unergative  Weak Adjective Collateral  Demonstrative  Possessive  Post-positive Adverb Genitive  Conjunctive  Flat  Prepositional  Pronomial Pronoun Demonstrative  Disjunctive  Distributive  Donkey  Dummy  Formal/Informal  Gender-neutral  Gender-specific  Inclusive/Exclusive  Indefinite  Intensive  Interrogative  Objective  Personal  Possessive  Prepositional  Reciprocal  Reflexive  Relative  Resumptive  Subjective  Weak Preposition Inflected  Casally modulated Conjunction Determiner Article  Demonstrative  Interrogative  Possessive  Quantifier Classifier Particle Discourse  Modal  Noun Complementizer Other Copula  Coverb  Expletive  Interjection (verbal)  Measure word  Preverb  Pro-form  Pro-sentence  Pro-verb  Procedure word