10. The computer readable medium of claim 9 , wherein identifying a second controlling keyword based on the tiebreaker rule comprises: determining, for each keyword among the two keywords, a selection score for the keyword based on a quality score for the keyword and a bid for the keyword; and identifying the second controlling keyword based on the selection scores for the two keywords.
9. The computer readable medium of claim 8 , wherein the instructions cause the one or more data processing apparatus to perform operations comprising: receiving a second content item request that specifies a second phrase of one or more words; determining that two keywords in a same content distribution campaign are both eligible to be selected as a controlling keyword for the second content item request; identifying a tiebreaker rule based on data associated with the two keywords; identifying, from the two keywords, a second controlling keyword based on the tiebreaker rule; and providing, in response to the second request, data associated with the second controlling keyword.