1. Resembles thymine, but easily interconverts to tautomeric form that pairs with guanine.

2. 5-Arylazo-8-quinolinols exhibit azo–hydrazone tautomeric equilibria but their complexes exist only in the azo form. β-Alkoxycarbonylethyltin alizarinates are somewhat different from other complexes.

3. Allelotropy: The existence in a tautomeric substance of the two isomeric forms in such a condition that either form readily passes over into the other.

4. The invention relates to amidine derivatives of formula (I), to the tautomeric and isomeric forms and salts thereof, to a method for producing the same and to the use of said derivatives for producing a pharmaceutical.

5. N, N -Dimethyl- N ′- (1-nitro-9-Acridyl)propane-1,3-diamine (Ledakrin® or nitracrine), which exhibits distinctive anticancer activity, and its three nitro isomers can potentially exist in two or three tautomeric forms (amino, imino, aci).