Symmetry creates balance in drawings and is an important element to understand when drawing a wide range of subjects. People, animals, vehicles and endless other objects have symmetrical structures. Artists often create balanced and harmonious compositions using symmetrical guidelines for placing shapes on the picture plane. The basics of drawing symmetrically can be learned using a simple step-by-step process.
Lightly sketch a vertical line down the center of the paper using a ruler. This will serve as the axis over which one side of your drawing mirrors the other. Think of how, when looking at someone's face, the left and right sides are generally the same. If you simulated a line down the middle of the face going between the eyes and over the nose and mouth you could choose one side, mirror it, and see basically the same whole face. This vertical axis line will serve as that mirror.
Draw half of the drawing on one side of the line. If you are drawing a butterfly, for example, draw one wing and one half of the body.
Locate an important point on your drawing, such as the center of the eye or tip of a wing. Measure its shortest distance to the axis line. Measure that same distance horizontally across on the opposite side of the axis line and mark that point. This point serves as the reference for where the corresponding point is located to indicate, for example, the center of the other eye or tip of the other wing. Duplicate this operation several times for other important locations on your drawing, such as the edge of the head or big shapes on the wing. The more of these points you make, the more frames of reference you will have for accurately drawing the other side of your picture.
Sketch in the remainder of the drawing using the framework of corresponding points. While doing so, it can be helpful to look at the axis line with your eyes while drawing. Though this may seem awkward at first, it allows your hand-eye coordination to better symmetrically copy what you see from one side to the other. For some artists, this is more effective than looking back and forth from one side to the other. Experiment to see which works better for you.
Try drawing and copying sections instead of drawing one whole side and duplicating it. Some artists experience this as a more balanced methodological process. Some drawings require much more sophisticated symmetry techniques. For example, drawing three-dimensional objects at three-quarters view will require studying perspective and practicing life-drawing.