These general rules are merely given as guides

The mouth is considered to be the most important feature of all in regard to the expression. This can be easily seen by an old test; covering all the face, showing only the eyes, and it will be almost impossible to tell whether the expression is a smile or a frown. If the mouth alone is exposed, there is no difficulty in making the decision. When smiling, the corners of the mouth turn upward, the lips part slightly, and the lines or dimples at either side curve outward. To express sorrow, the corners droop, the lips close and the lines become straight and severe.

These general rules are merely given as guides and are not intended to be arbitrary, for of course in each face such conditions may alter somewhat, according to the individuality exhibited.