Assign locant 1 to an atom adjacent to a bridgehead, and move further away from it so that lower locants are given to:

1. The heteroatoms together.

2. The heteroatoms according to priority.

3. The carbon atoms that are in the fusion position.

4. The indicated hydrogens. In peripheral numbering, the bridgehead carbons are not numbered, although they are when heteroatoms coincide in that position.

Bridgeheads are indicated by the locant of the atom that precedes them followed by the letter a (2a, 3a, 4a). In case of heterocycles with several neighboring bridgehead carbons, 3a, 3b... are used.

The peripheral numbering of the condensed heterocycle fulfills the mission of locating substituents, indicated hydrogens or hydros.