We study whether sequential ad content - display ads and landing page ads - act as substitutes or complements using a field experiment with a semiconductor firm. Ad headlines emphasizing product quality or transactional ease were randomly assigned across two ad placements in eight markets. Using pre-experiment organic traffic to infer prior knowledge about the advertised products, we find that the relationship between the two contents varies with prior information. In low-information markets, two quality-focused contents across placements showed substitutability, consistent with ad content serving as a source of information. In high-information markets, they exhibited complementarity, suggesting their preference-matching role. Decomposing quality-focused content reveals that content emphasizing horizontal match drives information provision, whereas content highlighting vertical quality supports preference matching. Our findings suggest that even coarse, market-level coordination of sequential ad content substantially improves online customer acquisition - lowering cost per action by 37%.
