Editing Solids and Surfaces - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 201

1. Open the drawing A3D_07_02.DWG . This drawing contains the primitive
solids created in Exercise 5.4 of Chapter 5 , 3D Primitives and Conver-
sions .
2. Apply the UNION command and select all the boxes, the two pyramids,
the rear cylinder and cone, and the torus. All these objects become a
single one:
3. To create the buttresses, apply the SUBTRACT command, select all the
six larger wedges, press the Enter key, and select all six smaller wedges.
The result is a single object for all buttresses, but we don't need these as
separate objects: