Methods, systems, and apparatus, including computer programs encoded on computer storage media, for evaluating substitute terms. One of the methods includes selecting a first term and a second term. A first co-occurrence frequency is determined for co-occurring terms in search queries that include the first term. A first vector is generated for the first term using the first co-occurrence frequencies. A second co-occurrence frequency is determined for the co-occurring terms in the search queries that include the first term adjacent to the second term. A second vector is generated for the second term using the second co-occurrence frequencies. A score for the second term as a context for a substitution rule based on the first term is computed, wherein the score is based on a comparison between the first vector and the second vector.