This Author's Guide, along with the sample article and sample book distributed with the PreTeXt source, provide a wealth of examples of how to author in PreTeXt. However, at some point, you will undoubtedly encounter a situation where some of your text fails to appear in your output or xsltproc produces an error. Those are good moments to start investigating the formal specifications of the PreTeXt vocabulary, as most likely you tried to use something in a way incompatible with those specifications. This chapter will help you understand and work with the formal specification of PreTeXt.