Often simply called a superlative, a superlative adjective states an extreme in a comparison. Many superlatives add "est" to the end of the adjective, at times changing the interior spelling as well. Some superlatives, however, are formed by adding the word "most" and creating an adjective phrase. A superlative adjective usually indicates the upper, but occasionally the lower, extreme of a comparison involving three or more nouns. If the specific objects of comparison are not explicitly stated, it is assumed the superlative adjective is the upper extreme for every object in the general category of comparison.
Like other adjectives, superlative adjectives describe nouns, but they do so in a comparative context. For example, in the sentence, "That car is the fastest I have driven," the word "fastest" is the superlative adjective. It indicates a comparison between the stated car and any other car the speaker has driven.
Although many superlatives are created by adding "est" to the end of the adjective, there are strict grammatical rules that govern their spelling. Words that end in a short vowel then a consonant, such as "big," double their ending constant before applying the suffix: "biggest." This change is necessary to retain the short "i" sound of the vowel in the original word since the addition of another vowel directly after the "g" would change the "i" to a long sound. Several adjectives have completely different spellings in their superlative form and simply have to be remembered. For example, the superlative for "good" is "best," whereas the superlative for "far" is "furthest" and "bad" is "worst."
Additionally, words that are two or more syllables and end in "y" require a spelling change from "y" to "i" before the suffix can be added. For example, "funny" becomes "funniest." If the multi-syllable word does not end in "y," however, there is no spelling change and the phrase "the most" is added to the beginning. For example, "amusing" changes to "the most amusing." One of the most common mistakes among English speakers is to confuse which adjectives require the suffix versus the beginning phrase or to use both with one adjective: "the most funniest."
In formal and academic writing, superlative adjectives should only be used when comparing specific factual items. Using a superlative adjective in general terms weakens arguments in formal rhetoric. For example, in the sentence "that was the best book," the statement implies the book was the best out of every book ever created. Since the writer cannot know every book in creation, the argument lacks validity.