How do the two numbers in a crown ether name describe its ring structure?

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heterocyclic compound There are many different sulfur-containing heterocycles. One of the best known is thiophene, C₄H₄S, derivatives of which occur as plant pigments and as other natural products such as biotin.

In the nomenclature of crown ethers, the two numbers provide specific information about the macrocyclic ring structure:

The first number indicates the total number of atoms present in the macrocycle (the large ring).1
The second number indicates the total number of heteroatoms (typically oxygen) contained within that ring.12

For example, in the compound 18-crown-6, the "18" signifies that the macrocycle consists of 18 atoms in total, while the "6" indicates that 6 of those atoms are oxygen. This naming convention is essential for identifying the size of the cavity within the ring, which determines the crown ether's ability to selectively bind specific metal ions.21