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The azodyes form one of the largest groups of dyes, and derive their name from the presence of the characteristic chemical group —N = N—, known as the azo group.

The nitro group has a very important action mainly on account of the readiness with which it can be introduced into the molecule, but its effect is much less than that of the azo group.

The mixed azo compounds are those in which the azo group �N:N� is united with an aromatic radical on the one hand, and with a radical of the aliphatic series on the other.

In the azo group some colors are fugitive, others are moderately fast, and it is generally recognized that certain classes of the tetrazo compounds are distinctly faster than the ordinary diazo colors.