Illustrator CS5 introduces the Perspective Grid to aid in creating full perspective artwork. The Perspective Grid can define one, two or three vanishing points as needed. The vanishing points are controllable in an interactive fashion. Objects can be created from scratch in the tool, or already created objects can be put into perspective. Keep in mind that the Perspective Grid is an Illustrator tool. It will not be part of your final artwork, even if the tool is visible when the artwork is completed. The Grid can be activated or deactivated, the Snap to Grid can be turned on or off, and individual objects can be removed from the grid.
Click "View" from the menu bar and select "Perspective Grid / Hide Grid" to deactivate the grid. The keyboard shortcut is "Ctrl," "Shift," "I" (Windows) and "Cmd," "Shift," "I" (Mac).
Select "Snap to Grid" from the Perspective Grid menu to turn off the default Snap to Grid setting.
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Click on "Object" and select "Perspective / Release with Perspective." Releasing the object from the grid won't turn off any perspective changes done while working with the grid.