Drawing guides are invisible lines that are automatically generated as soon as a drawing plane becomes active during a drawing operation. Drawing guides are parallel with the active drawing plane X and Y axes and run through all end-points and mid-points of existing object edges, which lie in the same plane as the drawing plane. As you move the cursor in the vicinity of a drawing guide, it will become visible and you can then snap the cursor to any point on the guide. When drawing guides are used in conjunction with Direction Snaps or the Protractor they offer a powerful mechanism for locating perimeter lines in relation to existing object edges.
See also the Adding blocks and using the measure tool tutorial
For example to create a triangular window in a surface with the same height as an existing opening, first click on the Draw window tool and check that the perimeter shape is set to 'Polygon', switch the protractor on and then move the cursor to align approximately with the lower edge of the existing window which will automatically snap the cursor to the required drawing guide:
Click to place the window origin and move the cursor to snap to the drawing guide associated with the existing window upper edge, the protractor angle lock will automatically combine with the drawing guide:
Click to place the apex of the window and then place the next point in a similar fashion:
Completing the window perimeter:
Note: In some cases Drawing guides can interfere with attempts to draw to a particular point and in this case you can temporarily switch off Drawing guides using the check boxes in the Drawing Options panel in the bottom left of the screen.