Change Measurement Systems on the Fly
You probabaly already know that you can type into just about any of InDesign's measurement fields a value based on any measurement system InDesign understands. For instance, if the document uses pica for measurement, you can still specify height, width, and numerous other values in inches by typing X in (for inches [also " and i]) or YpZ (for pica). The same trick works for centimeters (cm), millimeters (mm), points (pt), and ciceros (c). All that you probably knew because people like me have been excitedly writing about it since InDesign 1.0.
What you may not know is that you can also change the global measurement system on the fly without ever going into the Units & Increments pane of Preferences.
It's a keyboard shortcut--four-fingered, true, but still typically faster than the Preferences route. On Mac, press CMD+OPT+SHIFT+U, and on Windows, CTRL+ALT+SHIFT+U, to cycle through the available measurement systems, in the order of the Units & Increments Ruler Units menus--Points, Picas, Inches, Inches Decimal, Millimeters, Centimeters, Ciceros, and Custom (measured in points). Each press of the keyboard shortcut will change both the horizontal and vertical rulers, as well as reflect in the applicable fields on the Transform, Control, and other palettes.