<\body> style files> One of the fundamental strengths of is the possibility to write your own style files and packages. The purpose of style files is multiple: <\itemize> They allow the abstraction of repetitive elements in texts, like sections, theorems, enumerations, etc. They form a mechanism which allow you to structure your text. For instance, you may indicate that a given portion of your text is an abbreviation, a quotation or ``important''. Standard document styles enable you to write professionally looking documents, because the corresponding style files have been written with a lot of care by people who know a lot about typography and aesthetics. The user may select a major style from the menu. The major style usually reflects the kind of document you want to produce (like a letter, an article or a book) or a particular layout policy (like publishing an article in a given journal). Style packages, which are selected from the menu, are used for further customization of the major style. For instance, the package enables European-style theorem numbering and the package contains macros for customizing the layout of sessions of the computer algebra system. Several packages may be used together. When you want to add your own markup to or personalize the layout, then you have to choose between writing a principal style file or a style package. In most cases, you will probably prefer to write a style package, since this will allow you to combine it arbitrary other styles. However, in some cases you may prefer to create a new principal style, usually by personalizing an existing style. This is usually the case if you want to mimic the layout policy of some journal. In this chapter, we will both explain how to write your own style packages and how to customize the standard styles. <\traverse> styles|customize/customize.en.tm> <\initial> <\collection>