Context effects refer to changes in consumer preference and choice responses when a new alternative is added to a choice set. This paper proposes a general scheme for classifying various context effects using newly defined share-ratio measures (SRM) and share-change measures (SCM). With these measures, we can also draw context maps and preference-substitutability maps that visualise the nature of context effects and positions of competing brands. These maps allow marketers to make positioning decisions that take advantage of positive context effects.