Chronicle Books, Artist's Manual, Angela Cair
"The composition or design of a picture can be compared to the foundations of a tall building. Without this solid, underlying framework, the whole thing would collapse. Even the simplest subject needs to be composed so that it fits effectively within the confines of the picture space and - most importantly - so that it emphasizes the message behind the work."
When I teach art I try to begin with the most fundamental points and build on that. Composition is the backbone of any artwork and should be considered before making any other choices. Through doing preliminary sketches one can deliberately think about where the emphasis or focal point of the subject matter will be, and how the various elements of the piece will bring the viewers eye to that point.
The composition should reflect the intent or message of the artist. Why did you chose this focal point? What is the overall feeling or experience you are trying to convey. For example. in a portrait, the eyes are the most likely focal point as the eyes are "the mirror of the soul". However in a painting of two or more people the relationship between the subjects is more likely the focal point. The choices in composition you will make to
emphasize these points will depend on what you want to express about the person or the relationships.
Composition should be carefully considered before choosing your medium, size or palette. Through doing preliminary sketches and coming up with the best composition for your work you will often naturally come to the conclusion that your piece should be large or small, and what medium & colors scheme will best enhance the composition and convey your intent. In this way, composition is the gateway between internal purpose and external expression.