Construction lines can be used to help in the creation of precise geometry. You can draw construction lines using the snaps and the protractor to lie at defined locations and then use these lines as a framework from which or to which various object geometries can be projected or attached.
At the building level, construction lines can also be snapped to the Z-axis to form vertical planes from which block geometry can be extruded.
Construction line snap points can be picked up at all levels regardless of the level at which they were drawn. For example all of these operations are possible:
Note: Because Construction lines are not part of the model (in the same way that outline blocks are not part of the model) they do not appear in the Navigator panel.
See also:
More information on how to work with construction lines can be found in the Shading using standard component blocks and Adding a roof, construction lines tutorials.