Centuries ago, whenever manuscript writers inserted special instructions or explanations into a book, they put them in red ink to set them off from the black used in the main text. (They used the same practice to highlight saints' names and holy days in calendars, a practice which gave us the term red-letter day.) Ultimately, such special headings or comments came to be called rubrics, a term that traces back to ruber, the Latin word for "red." While the printing sense remains in use today, rubric also has an extended sense referring to any class or category under which something is organized.