Another very valuable idea originated within the generative approach was that of using special features assigned to the constituents, and specifying constraints to characterize agreement or coordination of their grammatical properties. For example, the rule NP ® D N in Spanish and some other languages with morphologic agreement of determinants and the corresponding nouns incorrectly admits constituents like *unas libro. To filter out such incorrect combinations, this rule can be specified in a form similar to an equation:

NP(Gen, Num) ® D(Gen, Num) N(Gen, Num),

where Gen and Num are variables standing for any specific gender and any specific number, correspondingly. The gender Gen and the number Num of a determinant should be the same as (i.e., agree with) the gender Gen and the number Num of the consequent noun (compare Spanish la luna and el sol, but not *unas libro). Such a notation is shorthand of the following more complete and more frequently used notation:

NP ® D N .

The following variant is also used, where the same value is not repeated, but is instead assigned a number and then referred to by this number. Thus, stands for the value Gen and for the value Num as specified where these numbers first occur:

NP ® D N .

The same constraint can be expressed in an even shorter equivalent form, where stands for the whole combination of the two features as specified when the symbol first occurs:

NP ® D N .

Each feature of a constituent can be expressed with a pair: its name and then its value, e.g., “gender: Gen”. The rule above determines which values are to be equal. With each constituent, any number of features can be expressed, while different constituents within the same rule possibly can have different sets of specified features.

Another example of the correspondence between features of constituents is the following rule of agreement in person and number between the subject and the predicate of a Spanish sentence (the syntactic predicate is a verb in finite, i.e., in personal form):

S ® NP(Pers, Num) VP(Pers, Num).

It means that, in the Spanish phrase like yo quiero or ellas quieren, there is a correspondence between the values of grammatical persons Pers and numbers Num: in the first phrase, Pers = 1st, Num = singular, while in the second phrase, Pers = 3rd, Num = plural on the both sides.

The constraint-based approach using the features was intensively used by the Generalized Phrase Structure Grammar (GPSG), and now is generally accepted. The featured notation permits to diminish the total number of rules. A generalized rule covers several simple rules at once. Such approach paves way to the method of unification to be exposed later.

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