Each chapter in a Primer is divided into short topics designed to be read as two facing pages: to read them in this way, you must download the content as PDFs. You can download whole chapters by clicking on the PDF option next to the chapter title; however these files are quite large and you may find it more convenient to download each topic individually, by clicking on the PDF option next to each topic. You need Acrobat Reader 4 or later to view our PDFs. If you have difficulty downloading PDFs, you may find our help page useful; or you may download the individual topics in web page format by clicking on the Full Text link.