One of Illustrator’s most powerful features may be strikingly familiar: Edit→ Undo gives you a way to take back the goof you just made. But that’s so common these days; in Illustrator, Edit→Undo is a multiple undo — it makes you the Master of Time! You can take your artwork back through time, step by step, all the way to when you first opened the document! If you make a mistake (or several), just choose Edit→Undo (or press z+Z on a Mac, Ctrl+Z with Windows) repeatedly, until you get back to the way things were before they went so wrong. To redo the last thing you undid, select Edit→Redo (or press z+Shift+Z on a Mac, Ctrl+Shift+Z with Windows). (A time machine with a reverse gear — way cool.)
Imagine creating with wild abandon — moving points, changing colors and line thicknesses, running filters — because in Illustrator, you can change your mind after the fact. Think of what you could obliterate from time — the misplaced stroke that looks like a bad tattoo, the missed goal that kept your team from the playoffs, the blind date that went so horribly wrong — well, okay, it only works in Illustrator. At least it works somewhere.
You can take a graphic back to the way it was when you first opened it, with one exception: If you close a file and then open it again, you can’t go back to anything you did before you closed the file. You can, however, save a file and then go back to things that happened before you saved it — if you left the file open after you saved it. In that case, the only thing that can’t be undone is the Save command: You still have a file on your hard drive that exists exactly as it did when you hit Save. This method can be useful if you have to create multiple revisions of the same document.