15. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a pair of words; generating a word-form score for the pair of words based on a consistency of the pair of words with word-form rules, wherein a word-form rule indicates how words with a common portion can vary; determining that the pair of words is a synonym pair based at least on the generated word form score; and generating an alternative search query for a search query that includes one of the words of the pair of words using another word of the pair of words.
16. The system of claim 15 , wherein the operations further comprise: receiving one or more synonyms; and generating one or more word-form rules from the synonyms.