The grammatical category associated with comparison of adjectives and adverbs is degree of comparison. The usual degrees of comparison are the positive, which simply denotes a property (as with the English words big and fully); the comparative, which indicates greater degree (as bigger and more fully); and the superlative, which indicates greatest degree (as biggest and most fully). Some languages have forms indicating a very large degree of a particular quality (called elative in Semitic linguistics).
Formation of comparatives and superlatives 
Comparatives and superlatives may be formed morphologically, by inflection, as with the English and German -er and -(e)st forms, or syntactically, as with the English more... and most... and the French plus... and le plus... forms. Common adjectives and adverbs often produce irregular forms, such as better and best (from good) and less and least (from little/few) in English, and meilleur (from bon) and mieux (from the adverb bien) in French.
Comparative and superlative constructions 
Comparatives are often used with a conjunction or other grammatical means to indicate with what the comparison is being made, as with than in English, als in German, etc. In Russian this can be done by placing the compared noun in the genitive case. With superlatives, the class of things being considered for comparison may be indicated, as in the best swimmer out of all the girls.
Languages also possess other structures for comparing adjectives and adverbs; English examples include as...as and less/least....
Usage when considering only two things 
In many languages, including English, traditional grammar requires the comparative form to be used when exactly two things are being considered, even in constructions where the superlative would be used when considering a larger number. For instance, May the better man win would be considered correct if there are only two individuals competing. However this rule is not always observed in informal usage; the form May the best man win will often be used in that situation, as it would if there were three or more competitors involved.
Rhetorical use of unbalanced comparatives 
In some contexts, such as advertising or political speeches, absolute and relative comparatives are intentionally employed in a way that invites a comparison, and yet the basis of comparison is not established. This is a common rhetorical device used to create an implication of significance where one may not actually be present. Although such usage is common, it is sometimes considered ungrammatical.
- Why pay more?
- We work harder.
- We sell for less!
Comparison in English 
English, because of the complex etymology of its lexicon, has two parallel systems of comparison. One involves the suffixes -er (the "comparative") and -est (the "superlative"). These inflections are of Germanic origin, and are cognate with the Latin suffixes -ior and -issimus. They are typically added to shorter words, words of Anglo-Saxon origin, and borrowed words that have been fully assimilated into the English vocabulary. Usually the words that take these inflections have fewer than three syllables. This system contains a number of irregular forms, some of which, like good, better, best, contain suppletive forms. These irregular forms include:
|little||smaller, less(er)||smallest, least|
More and most 
The second system of comparison in English appends the grammatical particles more and most, themselves the irregular comparatives of many/much, to the adjective or adverb being modified. This series can be compared to a system containing the diminutives less and least.
This system is most commonly used with words of French or Latin derivation; adjectives and adverbs formed with suffixes other than -ly (e.g. beautiful); and with longer, technical, or infrequently used words. Knowing which words fall into which system is a highly idiomatic issue in English syntax. Some words require the suffixing system: e.g. taller is required; *more tall is not idiomatic English.
Some words (e.g. difficult) require more and most. Some words (e.g. polite) can be used with either system; curiously, while polite can go either way, the derived word impolite requires more and most.
The general rule is that words with one syllable require the suffix, words with three or more syllables require more or most and words with two syllables can go either way.
Absolute adjectives 
A perennial issue in English usage involves the comparison of so-called "absolute" adjectives, also called ungradable adjectives, which logically do not seem to admit of comparison. There are many such adjectives — generally adjectives that name qualities that are either present or absent: nothing is *"more Cretaceous" or *"more igneous" than anything else.
Other examples include perfect, unique, and parallel, which name qualities that are inherently superlative: if something is perfect, there can be nothing better, so it does not make sense to describe one thing as *"more perfect" than something else; if something is unique, in the sense of "one of a kind", something cannot be *"very unique", or *"more unique" than something else. See also tautology (rhetoric) and pleonasm.
In general, terms like perfect and parallel cannot ever apply exactly to things in real life, so they are commonly used to mean nearly perfect, nearly parallel, and so on; and in this (inexact) use, more perfect (i.e., more nearly perfect, closer to perfect) and more parallel (i.e., more nearly parallel, closer to parallel) do seem to make sense.
- Tom McArthur, ed. (1992) The Oxford Companion to the English Language, Oxford University Press. ISBN 0-19-214183-X
- Trenga, Bonnie (12 August 2008). "Comparatives Versus Superlatives". Grammar Girl. Quick and Dirty Tips.
- For a sense of the word that could allow a qualifier, see J.D. Salinger. "For Esmé—With Love and Squalor". Retrieved 21 Feb 2013.: "And as I look back, it seems to me that we were fairly unique, the sixty of us, in that there wasn’t one good mixer in the bunch."