A 'qualitiative' adjective is one whose meaning compares in English. Qualitative adjectives also occur in predicate position, form adverbs and nouns. For example, громкий may occur in predicate position (Его голос громок 'His voice is loud'), it may be used as an adverb (Она говорить громко 'She talks loudly') and has a nominal form (промкость 'loudness, volume'). So, громкий has a set of comparative forms: громче, более громкий, самый громкий.
Adjectives without these properties (relational adjectives) may not be compared. For example, the adjectives биологический 'biological', государственный 'state, federal', городской 'city, urban' do not have these properties and therefore are not susceptible to comparison. Even in English we can't say 'our decay is more urban than yours' or 'our government is more federal than yours'. But we can say 'louder', 'loudly' and 'loudness'.
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