To use gridlines
Click the dimensions pill found at the bottom left of the proofing screen and select use gridlines. Or, click the gridlines icon found in the magic tools.
Click and drag from the gridlines area to draw a vertical or horizontal gridline.
To change the unit of measurement, in the dimensions pill click the units and select a different option (pixels, millimeters, inches, etc)
Use the keyboard shortcut g to turn the gridlines on/off.
Click here to learn more about our ruler tool.