Guides are lines that you drag onto the document from the rulers. They help you place and align objects more precisely. You can also use guides to measure the size of page elements, or emulate the folds (visible areas) of web browsers.
To help you align elements, you can snap elements to guides, and snap guides to elements. (Elements must be absolutely positioned in order for the snap feature to work.) You can also lock guides to prevent them from being accidentally moved by another user.
Create a horizontal or vertical guide
Show or hide guides
Snap elements to guides
Lock or unlock all guides
View and move a guide to a specific position
View the distance between guides
Press Control (Windows) or Command
(Macintosh) and hold the mouse pointer anywhere between the two
guides.
Emulate the fold (visible area) of a web browser
Remove a guide
Change guide settings
Select View > Guides > Edit
Guides, set the following options, and click OK.