Communication: The influence of the alignment and density of dye molecules on the spectroscopic properties of aggregates can be studied particularly well in J-aggregates of 1,7-bis(dimethylamino)heptamethine (BDH(+)). Four different arrangements of BDH(+) molecules can be obtained without changing the molecular structure simply by using different counterions. The relationship between structure and linear spectroscopic properties-including the color of crystals containing BDH(+)-is discussed.