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Fused heterocycles are formed by two cycles that share a bond, the condensation bond. One of the heterocytes is considered principal, while the other is called annexed. The name is built starting with that of the attached heterocycle, followed by a bracket where the bond by which both cycles condense is specified and ending with the name of the base heterocycle.

To name a condensed heterocycle we must know the rules that allow us to choose the base heterocycle, as well as the peripheral numbering to give the position of possible substituents. In addition, the attached heterocycle is numbered and the base bonds are listed to specify the condensation bond.

The base component retains the name as a heteromonocycle; the name of the attached component is that of the corresponding heteromonocycle ending in "o". There are some exceptions: furo (furan); thiene (thiophene); pyrido (pyridine); pyrimido (pyrimidine); imidazo (imidazole).