From the publisher's website: "The purpose of this document is to expand the basic idea of the thief as presented in the OSRIC basic rules. This is not an official rules expansion though this document was written specifically for use with the OSRIC rules. With a little work, it can also be used with other sword and sorcery type games popular in the 1970s and 1980s. The enterprising GM may use this material as written, in whole or in part, or simply use this material as inspiration for his or her own unique interpretation of the thief class."