With the advent of text-processing systems came new types of markup and new types of processing. When prepared for reading, either on screen or on paper, documents are marked up scribally. But, when stored in electronic files, documents may be marked up scribally or with special electronic types of markup designed for processing by computer applications. One uses procedural markup to indicate the procedures that a particular application should follow (e.g., .sk to skip a line), descriptive markup to identify the entity type of the current token (e.g. <p> for paragraphs), referential markup to refer to entities external to the document (e.g. — for an em dash), and metamarkup to define or control the processing of other forms of markup (e.g. <! ENTITY dem “Association for Computing Machinery”> to define the referential markup &acm;).