Syntactic function, case name, and case marking are three interrelated topics that taken together determine how a noun should be. A noun-type word can have different syntactic functions. For the purpose of illustration, it is useful to examine three such functions: subject of a verb, object of a verb, and object of a preposition. There are three cases for Arabic nouns. They are nominative case, accusative case and the genitive case. Case marking means providing a clue indicating the case or function of a noun. There are two ways to mark the case of Arabic nouns: diacritics and letters. Whereas diacritics are often by convention left out of written Arabic, case-marking letters cannot be left out of written words because letters are never left out of written Arabic. Letters are used for case-marking nouns in the dual and nouns in the sound masculine plural.