• Every atom and bond in S is also present in A. H atoms in S must be explicit to be considered, but explicit H atoms in S may be explicit or implicit in A.
• Every bond in S has the same order in A.
• Corresponding atoms in S and A have the same charge.
• Any isotopes or stereochemical configurations indicated in S are the same in A.

Compound A contains a skeleton S when:

• Every atom and bond in S is also present in A. These atoms and bonds in A are called "skeletal". H atoms in S must be explicit to be considered, but explicit H atoms in S may be explicit or implicit in A.
• Each skeletal bond in A has the same order as or a higher order than the corresponding bond in S. (Double and aromatic bonds are treated as identical.) For example, cyclohexene contains the cyclohexane skeleton, but not vice versa.
• For every skeletal C atom in A that is attached to another C atom, both the bond and the latter C atom are skeletal. For example, ethoxybenzene contains both the benzene and the phenol skeletons, but toluene does not contain the benzene skeleton; and 2-Azabicyclo[3.1.0]hexane does not contain the piperidine skeleton, but 1-azabicyclo[3.1.0]hexane does.
• Any charges, isotopes, or stereochemical configurations indicated in S may differ or be absent in A, and vice versa.