112 CHAPTER 5 The building blocks of natural language rule statements 2. after some verbs, including `check', `that' may be replaced by `whether' or `whether or not'; the last example above would then read "The flight attendant checked whether all passengers' seat belts were fastened" or "The flight attendant checked whether or not all passengers' seat belts were fastened". 5.4 DETERMINERS One or more determiners can be used before a noun to provide some information as to which (or how many) instances of the noun's concept are being referred to. As we shall see in Section 5.5, adjectives also do this. While there are various differences between determiners and adjectives, none of them applies to all determiners or all adjectives, so I will not cover them in this book. Indeed, there is no one definitive list of determiners agreed on by all linguists. Fortunately, only a few of the many determiners available in the English language are used in rule statements (none is used in fact types) so we can list them all. Without doubt, the most commonly used determiners are articles. 5.4.1 Articles There are two types of articles: 1. the indefinite articles: `a' and `an'; 2. the definite article: `the'. An indefinite article is used before a singular noun when referring to an unspecified single instance: for example, `a customer', `an address', `a product', `an order'. The definite article `the' can be used before a singular or plural noun. I find that it is frequently misused: it can only be used legitimately before a singular noun when referring to 1. the only instance of that concept that exists, as in "the Earth"; 2. the only instance of that concept that exists in the context already established, as in "the defendant" (in a court case); 3. (where an instance of the concept has already been referred to) the instance already referred to (by inclusion of `same' after `the'), as in "the same day"; 4. (where two instances of the concept exist--or exist in the context already established--and one has already been referred to) the other instance (by inclusion of `other' after `the'), as in "the other party"; 5. the only instance of that concept that meets the criterion or criteria specified: a. by one or more attributive adjectives or nouns between `the' and the noun, as in "the eldest son", "the hardware shop", b. in a qualifying clause (see Section 7.2.7 in Chapter 7) after the noun, as in "the person named in the application", c. in both a qualifying clause and one or more attributive adjectives or nouns between `the' and the noun, as in "the destination city specified in the flight booking request". Similarly `the' can only be used legitimately before a plural noun when referring to 1. all instances of a concept, as in "the Australian states"; 2. all instances of a concept that exist in the context already established, as in "the people" (in a constitution);