|Document Window: Rulers and Guides|
To add rulers to the image view area, click the Ruler icon in the Toolbar, or choose Show Rulers (Command-R) from the Tools menu.
To set the ruler units, click the Units icon in the Toolbar or set the Ruler Units in the General tab of the Preferences window.
When rulers are visible, you can add guides to the view window.
To add a vertical guide, click in the ruled area of the horizontal ruler and drag up into the area above the ruler.
To add a horizontal guide, click in the ruled area of the vertical ruler and drag into the area to the left of the ruled area.
To move a guide, click and drag on its handle.
To remove a guide, click on its handle and drag it off the ruler.