While continuing to hold the mouse button down, drag the O to the middle of the Front window. This should give you vertical and horizontal lines. Notice that your rulers show a 0 at the 2 lines. Press Command D to refresh the screen if the 0 is not showing at the two origin lines. Setting Guidelines Now click on the pop-up button that had the origin point and this time drag the G to the Front window.
G stands for guidelines.
You can drag as many as you want into your window. These are used to line up objects, vertices and direction points. Click on either the vertical or horizontal rulers and drag into the window. Notice that you now have more guidelines. Delete all the guidelines by going to the Options menu and select Clear All Guides .