7. The system of claim 1 , wherein the one or more computers are further configured to select, as a set of matching labels, initial labels that each include at least one same word.
1. A system, comprising: a data store storing label data that specifies a set of initial labels for a non-text content item and a resource identifier that specifies, for each initial label, a web page to which the initial label is associated, wherein the non-text content item is associated with each of a plurality of web pages, and wherein each initial label includes one or more words; and one or more computers coupled to the data store, the one or more computers configured to: generate initial label groups for sets of matching web pages, each set of matching web pages including two or more matching web pages; group, for each set of matching web pages, initial labels that are associated with the set of matching web pages into a separate initial label group that corresponds to the set of matching web pages; generate initial label groups for sets of matching labels, each set of matching labels including two or more initial labels; group, for each set of matching labels, initial labels that are associated with the set of matching labels into a separate initial label group that corresponds to the set of matching labels; and select, as final labels for the non-text content item, n-grams of one or more words that are included in at least a threshold number of the separate initial label groups.