If you make your Rulers visible (View>Rulers), and click and drag from the vertical Ruler onto you work area, you get a vertical Guide. Do the same from the horizontal Ruler and you’ll get a horizontal Guide. But is there a way to drag a vertical Guide from the horizontal Ruler or a horizontal Guide from the vertical Ruler? Sure! As you click on either Ruler, hold down the Option key (Alt on PC) and the Guide will flip around sideways as you drag. This same trick works in InDesign.
On behalf of layers magazine.