1. A method performed by one or more computers, the method comprising: receiving a search string from a user device; determining a measure of query completeness for the search string; comparing the measure of query completeness to a threshold measure of query completeness; and providing one or more specific query suggestions to the user device, selected from a plurality of specific query suggestions for the search string, when the measure of query completeness exceeds the threshold measure of query completeness; or providing one or more general query suggestions to the user device, selected from a plurality of general query suggestions for the search string, when the measure of query completeness does not exceed the threshold measure of query completeness.