GEMs, or General Equivalence Mappings were created by the National Center for Health Statistics, CMS, and other industry groups. Our book contains the official data. The reason that many of the codes are unspecified is because the ICD-9 codes are not very specific, so it would be incorrect to map them to codes that don't match. As such, we instruct coders to use GEMs as step one, then turn to the suggested code in the Tabular List. It is here where someone with a little knowledge of the code set can confidently browse and select a more specific code, if available.