In linguistics, especially within generative grammar, phi features (denoted with the Greek letter φ 'phi') are the morphological expression of a semantic process in which a word or morpheme varies with the form of another word or phrase in the same sentence. This variation can include person, number, gender, and case, as encoded in pronominal agreement with nouns and pronouns (the latter are said to consist only of phi-features, containing no lexical head). Several other features are included in the set of phi-features, such as the categorical features ±N (nominal) and ±V (verbal), which can be used to describe lexical categories and case features. Phi-features are often thought of as the "silent" features that exist on lexical heads (or, according to some theories, within the syntactic structure) that are understood for number, gender, person or reflexivity. Due to their silent nature, phi-features are often only understood if someone is a native speaker of a language, or if the translation includes a gloss of all these features. Many languages exhibit a pro-drop phenomenon which means that they rely on other lexical categories to determine the phi-features of the lexical heads. Agreement on nouns Chomsky first proposed that the N node in a clause carries with it all the features to include person, number and gender. In English, we rely on nouns to determine the phi-features of a word, but some other languages rely on inflections of the different parts of speech to determine person, number and gender of the nominal phrases to which they refer. Adjectives also carry phi-features in some languages, however, they tend to agree in number and gender but rarely for person. Number The grammatical term number is the name of the system contrasting singular and plural. In English, number agreement is not expressed through agreement of verbal elements like they are in other languages (though present tense verbs do agree in number with third person subjects). This is partly because English is a language that requires subjects and the subjects in English overtly express number. Instead, English number is a phi-feature that is inflected on nouns when the nominal phrase is plural. The most common in English is -s inflected on nouns that are plural:- Ducks, fridges, baseballs, cups, books, mirrors, cars, buildings, clowns, bridges, creams....Some cases of plurality in English require inflection within the noun to express the phi-feature of plurality:- Men, women, mice, teeth....Neither verbs