“The most obvious distinction between various kinds of lines is the distinction between straight lines and curves. Straight lines will always express rigidity and stiffness while curves will suggest some sort of growth or motion; but straight lines vary in expression according to their position and direction. The horizontal line is always suggestive of repose; it is the line of resting water, of the earth of alluvial plains, of everything that has reached a state of equilibrium. The vertical line is a line of stability, of direct opposition to the force of gravity, of strength and vigor. Most compositions in which the sentiment of restfulness and enduring peace is to be expressed are built on a combination of verticals and horizontals. Oblique straight lines vary in expression according to their combination with other lines and may express anything from tottering to vigorous thrusting; but they nearly always express some form of motion.
As straight lines express strength, so curves express softness, and the softest of curves are those approaching the circular or made up of sections of circles. An infusion of straightness into a curve will give it stiffness and vigor, and the most lively and elastic curves are those approaching straightness at one end and curving more and more rapidly toward the other… All these characters of lines may be the result of association or they may have some deeper reason, but they are there, in the lines themselves, without regard to what the lines may be used to represent, and are among the most valuable means of artistic expression.” Cox Concerning Painting p130