The following criteria apply until one unequivocally decides:
1. A nitrogen-containing compound.
2. The component containing the highest priority heteroatom (without nitrogen).
3. The component that contains the largest of the rings.
4. The component containing the largest number of heteroatoms of any type.
5. The component containing the greatest variety of heteroatoms.
6. The component containing the largest number of most preferred heteroatoms.
7. The component that, before condensation, has minor locants for its heteroatoms.
8. Rule 7, considering the heteroatoms in order of priority.

In the following fused heterocycles the base component is represented in blue.

choice of base component

In the first molecule the base component is chosen by "rule 1", nitrogen-containing cycle.

In the second molecule, rule 1 is not applicable, since none of the cycles has nitrogen, so we go to rule 2. Since oxygen has priority over sulfur, we chose furan as the base component.

In the third molecule the first two rules fail, but the third decides since the rings are of different sizes, choosing the six-membered cycle as the basis.

In the fourth molecule we need to reach rule 4 to obtain the base heterocycle, this being the one with the largest number of heteroatoms.