Elementary Art Lessons, back to the basics.

In representational art, realism, or classical, all depend on some elementary art lessons to achieve what is considered a "real looking" three dimensional object onto a two dimension piece of canvas.

It is called form, or modeling. It's what is done on the canvas to mimic what the viewer is seeing. It is something that is an integral part of realism, without it, you get into other styles of oil painting , even abstract or cubism.