Righthand head rule
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In generative morphology, the righthand head rule is a rule of grammar that specifies that the rightmost morpheme in a morphological structure is almost always the head in certain languages. What this means is that it is the righthand element that provides the primary syntactic and/or semantic information. The projection of syntactic information from the righthand element onto the output word is known as feature percolation. The righthand head rule is considered a broadly general and universal principle of morphology. In certain other languages it is proposed that rather than a righthand head rule, a lefthand head rule applies, where the lefthand element provides this information.
In derivational morphology (i.e. the creation of new words), the head is that morpheme that provides the part of speech (PoS) information. According to the righthand head rule, this is of course the righthand element.
For instance, the word 'person' is a noun, but if the suffix '-al' were added then 'personal' is derived. 'Personal' is an adjective, and the righthand head rule holds that the PoS information is provided by the suffix '-al', which is the righthand element.
The adverb 'personally' is derived from 'personal' by adding the suffix '-ly'. The PoS-information is provided by this suffix which is added to the right of 'personal'.
The same applies to the noun 'personality', which is also derived from 'personal', this time by adding the nominal suffix '-ity' to the right of the input word. Again the PoS-information is projected from the righthand element.
The three above examples may be formalized thus (N=noun, ADJ=adjective, ADV=adverb):
- personN + -alADJ = personalADJ
- personalADJ + -lyADV = personallyADV
- personalADJ + -ityN = personalityN
They are all instances of the righthand head rule, which may be formalized as:
- Ax + By = Cy
The righthand head rule in inflectional morphology
The righthand head rule may also be applied to inflectional morphology (i.e. the addition of semantic information without changing the word class). In relation to inflectional morphology, the righthand head rule holds that the rightmost element of a word provides the most essential additional semantic information.
For example, the past tense form of 'play' is created by adding the past tense suffix '-(e)d' to the right. This suffix provides the past tense feature which is also the main additional semantic content of the output word 'played'.
Likewise, the plural form of 'dog' is created by the addition of the plural nominal suffix '-s' to the right of the input. Thus 'dogs' inherits its plurality feature from the suffix.
The same thing goes for the comparative form of the adjective 'ugly'. 'Uglier' is created by the addition of the comparative suffix '-er' to the right, thus receiving its comparative feature from the suffix.
Formalizing the examples shows that the underlying principle of inflection is basically the same as the righthand head rule (INF=infinitive, P=past tense, SG=singular, POS=positive, COM=comparative):
- playINF + -(e)dP = playedP
- dogSG + -sPL = dogsPL
- uglyPOS + -erCOM = uglierCOM