a) simple volumes(building blocks)
b) horizon
c) construction lines

Construction in drawing refers to the process of blocking in a complicated object/figure/scene by breaking it down to a combination of simple volumes, e.g. spheres, cuboids, cylinders, cones and pyramids.

Construction, when used alone, often leads to very stiff, mechanical looking results. Results will appear more dynamic if construction is used as a second step on top of gesture drawing.

In the example here, construction was done in a Perspective Projection without gesture.

further reading:

• Form And Proportion
• Drawing Fundamentals Construction