Hard-copy handbooks and similar reference documents are considered by some to be passé in today's world of computer-based design and virtual reality. None-the-less, much of the knowledge gained over the years as well as new information is documented in handbooks, manuals, data books, and so forth. Guidance, rules-of-thumb, lessons-learned, and similar information, together with explanations make handbooks and other reference documents important resources for the engineer. Some older documents are being "digitized" for entry into computer data bases making it easier to search and update the information. Nearly all new documents are created in digitized form.