In this course, we will begin to explore value, which is the backbone of great painting. The most effective way to study and simplify value is to develop 'poster studies.' A poster study seeks to simplify the dominant values and shapes in a composition. Rather than rendering and blending, you leave the study in a more graphical, hard-edged form. This enables you to see the large shapes, masses, and dominant values clearly. By simplifying your composition, you are in the best position to judge its effectiveness and to anticipate challenges.

After introducing value and two basic poster studies, we will then take a look at the concept of 'key.' Key refers to the overall values being constrained to one side of the value scale or the other. A composition that is dominant in light values with subordinate dark notes is called "high key." A composition that is dominant in dark values with subordinate light notes is called "low key." And if the values group toward the center of the value scale, we will refer to this as "middle key." Key can help create a tone or mood for your composition, and  you can manipulate key to create the emotional tone you desire.

More info - https://www.skillshare.com/classes/Oil-Painting-for-Beginners-Value-Poster-Studies/967907582