Pages provides horizontal and vertical rulers to help you lay out text and objects in your document. The vertical ruler is always available for page layout documents. For word-processing documents like a letter or report, you need to enable the vertical ruler in Pages preferences.
You can set the ruler to use different increments (points, inches, centimeters), or show the tick marks as percentages. You can also place zero at the center of the ruler, which can be useful if you’re designing a page layout document.
Choose Pages > Preferences (from the Pages menu at the top of your screen).
Click Rulers.
Select the checkbox next to “Enable vertical ruler for documents with body text.”
Do one of the following:
Click 
in the toolbar, then choose Show Ruler (or Hide Ruler).
Choose View > Show Ruler (or Hide Ruler) from the View menu at the top of your screen.
When you set a unit preference for rulers in Pages, it applies to all documents. You can change the preference at any time, even if you have a document open.
Choose Pages > Preferences (from the Pages menu at the top of your screen), then click Rulers at the top of the preferences window.
Click the Ruler Units pop-up menu, then choose an increment (Points, Centimeters, or Inches).
To show the ruler marks as a percentage of the page, select the “Display the ruler units as percentage” checkbox.
You can place zero at the center of the ruler so that ruler units increase to the left and right.
Choose Pages > Preferences (from the Pages menu at the top of your screen) then click Rulers at the top of the preferences window.
Select the “Place origin at center of ruler” checkbox.
Tip: You can also use the ruler to set margins and tab stops, and you can drag alignment guides from the ruler to the page.