Project Gutenberg also publishes eBooks in well-known formats like HTML, XML or RTF. There are Unicode files too. Any other format provided by volunteers (PDF, LIT, TeX and many others) is usually accepted, as long as they also supply an ASCII version where possible. But a large scale conversion into other formats is handed over to other organizations. For example Blackmask Online, which uses Project Gutenberg's collections to offer thousands of free eBooks in eight different formats based on the Open eBook (OeB) format. Or Manybooks.net, which converts Project Gutenberg's eBooks into formats readable on PDAs. Or Bookshare.org, the main digital library for the visual impaired community in the US, which converts books from Project Gutenberg into Braille format and DAISY (Digital Audio Information System) format. What is entailed exactly, once copyright clearance is received? Digitization is done by scanning the book page after page to get "image" files. Then volunteers run an OCR (Optical Character Recognition) software to convert "image" files into text files. Then each text file is proofread (i.e. re-read and corrected) by comparing it to the "image" file or the original page of the print version. There is an average of 10 mistakes per page for a good OCR package and... many more mistakes if the quality of the scanner and the OCR package is not great. The book is proofread twice on the computer screen by two different people, who make any corrections necessary. When the original is in poor condition, as with very old books, it is keyed in manually, word by word. Some volunteers themselves prefer to type short texts, or works they particularly like. But most books are scanned, "OCRized" and proofread. Digitization in "text format" means a book can be copied, indexed, searched, analyzed and compared with other books. It is possible to search the content of the book with the "Find" button available in any browser and any software, without a specific search engine. Project Gutenberg provides a "Nearly Full Text" search (on the first 100 K of each file) using Google, with a database updated approximately monthly. It also provides a search of book metadata (author, title, brief description, keywords) as a participant in Yahoo!'s Content Acquisition Program, with a database updated weekly. (Please see the bottom of the Online Book Catalog.) In the Advanced Search, several fields can be filled: author, title, subject, language, category (any, audio book, music, pictures), LoCC (Library of Congress Catalog classification), filetype (text, PDF, HTML, XML, JPEG, etc.), and eText/eBook No. A field "Full Text" was recently added as an experimental feature.