Magnifying glasses are often used for viewing three-dimensional objects.
Again, a rigid three-dimensional frame can be rigidly fixed relatively to the earth by means of six links.
Hence any three-dimensional system can be reduced to a single force R acting in a certain line, together with a couple G in a plane perpendicular to the line.
In general, however, a three-dimensional system of forces cannot be replaced by a single resultant force.
The analytical reduction of a three-dimensional system can now be conducted as follows.