16. A system comprising: one or more computers and one or more computer-readable storage devices storing instructions that, when executed by the one or more computers cause the one or more computers to perform operations comprising: identifying a candidate n-gram that includes two or more consecutive terms of a search query; determining a first quantity of search results that (i) were identified as responsive to the search query, and (ii) have been selected by other users; determining a second quantity of the search results that (i) were identified as responsive to the search query, (ii) have been selected by the other users, and (iii) are associated with text in which the candidate n-gram occurs; determining a value using the first quantity and the second quantity; and classifying the candidate n-gram as a particular type of n-gram, from among multiple types of n-grams, based on the determined value.
16. A system comprising: one or more computers and one or more computer-readable storage devices storing instructions that, when executed by the one or more computers cause the one or more computers to perform operations comprising: identifying a candidate n-gram that includes two or more consecutive terms of a search query; determining a first quantity of search results that (i) were identified as responsive to the search query, and (ii) have been selected by other users; determining a second quantity of the search results that (i) were identified as responsive to the search query, (ii) have been selected by the other users, and (iii) are associated with text in which the candidate n-gram occurs; determining a value using the first quantity and the second quantity; and classifying the candidate n-gram as a particular type of n-gram, from among multiple types of n-grams, based on the determined value. 20. The system of claim 16 , wherein the particular type of n-gram includes a combination of n terms that has a different meaning in combination than the n terms do separately.