The exterior minor protein IX of adenoviruses (AdVs) is a frequent target of attachment of antigens and the modified AdVs are being used as potent vaccine platforms. The organization of protein IX is disticntly different between human adenoviruses (HAdVs) and non-HAdVs. The analysis of solvent accessibility, based on the near atomic resolution structures, suggests that the C-terminal residues of IX are more accessible in non-HAdVs (e.g., bovine adenovirus) than in HAdVs. Although the C-terminal fusions of IX are displayed on the capsid surface, they could disrupt the formation of tetrameric coiled-coils (4-HLXB) in HAdVs due to steric hinderance, thereby potentially affecting the capsid stability. Importantly, the parallel-antiparallel arrangement of helices seen in the 4-HLXB is not condusive for IX C-terminal fusions in HAdVs. In contrast, the parallel trimeric C-terminal coiled-coils in non-HAdVs are unlikely to be affected by the attachment of antigens and more efficiently displayed on the AdV surface.