Spelling Rules for Comparatives and Superlatives

Adjectives ending in -L double the L before taking -er and -est: crueller, cruellest.
One-syllable adjectives ending in letter -E take only -r in the comparative and -st in the superlative, such as in nicer-nicest.
Two-syllable adjectives ending in -Y turn the Y into I and then take -er or -est such as in happier-happiest. Adjectives ending in a stressed vowel between two consonants double the final consonant and then take -er or -est: thinner,thinnest. Watch out! There is an exception: old, older, oldest.
One more thing, letters Y, W and X are never doubled.
Now drag the adjectives to the right dustbin.

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