Creating Solids and Surfaces from 2D - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 100

Another innovation in Version 2013 is the difference between extruding and offsetting
faces. If the face is normally selected, it is extruded while maintaining the area, but
if the face is selected while keeping the Ctrl key pressed, the face is offset and the
adjacent face maintains its angle.
Exercise 4.2
From a 2D plan, we will create the volumetric representation of a street in a couple
of minutes by following these steps:
1. Open the file A3D_04_02.DWG . This simple drawing has several closed
areas, each with a number that represents the height of extrusion.
2. Before applying the command, orbit the drawing: