The Chemical Abstracts Service (CAS) maintains a registry system for all completely identified chemical compounds and substances. There are over 67 million chemical substances currently registered and one new substance is added to the registry every 9 seconds!
Each substance is assigned a unique number (referred to as a CAS Registry Number, or CAS RN). A registry number looks like this: 123456-78-9 where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. The registry number has no structural significance--it is simply used as an identifier.
The rules of chemical nomenclature frequently change and each substance may have several names. Such as:
trade or brand names (Tylenol)
generic or common names
trivial or semisystematic names (acetaminophen)
systematic or IUPAC names. (Acetamide, N-(4-hydroxyphenyl)- AND 4'-Hydroxyacetanilide)
BUT just one CAS registry number, 103-90-2. Even if other names are created for this compound, the registry number will remain unchanged. Searching by the Chemical Registry number eliminates a lot of confusion about chemical names.