The principles of Smile Design take into account that the smile is viewed in the context of and framed by the surrounding facial form.
The first consideration is what is the outline form of the patient's face, and consequently, what outline form do we want the teeth to have?
Once we have selected the shape of the teeth, we now have to decide how big or small they need to be. Each upper central incisor should be about 1/16th the width and height of the face.
The size of the front teeth should become progressively smaller as we proceed toward the back of the mouth in the ratio of 1.6 to 1.0 to 0.6 called The Golden Proportion.
Now that we know the shape and size of the individual teeth, we have several principles to help us align them in three dimensions in an arrangement which harmonizes with certain facial landmarks.
The dental midline of the upper front central incisors is parallel to the the facial midline and perpendicular to a line drawn through the pupils of the eye and corners of the mouth.
A line drawn through the biting edges of the upper front teeth will create an arc of curvature parallel to the curvature of the lower lip.
In broad strokes, this is the essence of how a smile can be in harmony with facial form.