Illustrator is, at its core, a PostScript interpreter. It's internal language is PostScript. So it's not all that difficult for other programs that do pretty much the same thing (FreeHand, CorelDraw, Canvas, Xara, etc.) to write an import filter to convert the content of a native .ai file into objects which are native to the importing program--and just throw away any Illustrator-specific stuff it doesn't understand. This was true up until the release of Illustrator 9. Since version 9, Illustrator's internal language is a superset of the PDF 1.5 language spec. PostScript has no support at all for many of the constructs that Illustrator now supports (transparency, etc). It's actually quite difficult for anyone -- even other Adobe teams for that matter -- to parse native .AI information since Illustrator 9. Since most Adobe apps don't need to access editable information -- they really just need an unflattened composite -- it's easier to just have Illustrator embed a PDF (1.5) version of the native Illustrator file which other apps can easily read and place. It's interesting to note that Adobe *IS* indeed started to include Illustrator's own graphics libraries into other apps, meaning that other apps WILL be able to parse Illustrator's native file format (think of these apps as having a built-in headless version of Illustrator). Adobe is doing this for apps where the editing of content would be important or necessary. For example, Flash CS3 and CS4 contains Illustrator's graphics libraries. Meaning that you can save your file from AI using the .ai file format WITHOUT checking the Include PDF Compatible File option, and Flash will be able to correctly parse the file. I believe that Fireworks can do this in CS4 (although I've experienced instability and constant crashes when doing so), and also, Adobe's new Flash Catalyst contains Illustrator's graphics libraries. At one time, the marketers at Adobe decided it clever to proclaim that "PDF is now Illustrator's native format." That ill-advised claim was mostly smoke-and-mirrors, and resulted in untold confusion among Adobe's customers. Hm. As I mentioned above, Illustrator's native file format IS indeed PDF -- but since it's a superset of the public ISO PDF language spec (it has to support things like Illustrator's 3D features, appearances, etc.), it's a version of PDF that only Illustrator itself can understand. But it's certainly PDF. There's no smoke and mirrors there. Also, as I mentioned above, the default behavior is that Illustrator embeds an unflattened composite of the file as a PDF file that CAN be read by other applications. This PDF is able to fully reproduce the art when printed (it's just not editable like the native version is). I don't think there's any confusion amongst customers. Most designers just place native .ai files into other Adobe apps and it just works. What Illustrator is doing "behind the scenes" really doesn't affect the average user at all. And designers like when things "just work". To specifically answer the original question in this thread, reading this article, What's In a File, should help. Mordy
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