In medicine and anatomy, planes are used to describe locations on the body. A plane in this sense is not a physical object, but instead an imaginary wall dividing the whole body or parts of the body in two. The sagittal plane separates the body vertically into right and left parts. The median plane lies directly in the middle of the sagittal plane, dividing the body into perfect halves. In fact, the median plane is often called the mid-sagittal plane.
Other planes in anatomy include the frontal plane, also called the coronal plane, and the horizontal plane, also known as the transverse plane. The frontal plane divides the body into front and back, while the horizontal plane divides the top of the body from the bottom. Using these three planes, sagittal, frontal, and horizontal, exact places of the body can be pinpointed, or the body can be discussed in terms of general areas. For example, the upper right quadrant of the body can be described as the area to the right of the median plane and above the mid-horizontal plane. The point of intersection at the exact midpoint of each of the three planes would lie at the very center of the inside of the body.
Using terms that describe positions relative to each of the planes can help explain movements and locations on the body. Medial and lateral refer to distances toward or away from the median plane. For instance, if a man were to stand upright and extend one arm straight out to the side, his shoulder would be medial, or lying closer to the median plane, than his hand. His hand would be lateral, or farther away from the median plane in relation to his shoulder. If he were to rotate his shoulder so that the arm is straight out in front of him and falls exactly along the sagittal plane parallel to the median plane, neither his shoulder nor his hand would be medial or lateral to the median because they would both be the exact same distance away on a perfectly parallel plane.
The other planes have relative positions that can be described as well. These include superior and inferior, as well as anterior and posterior. For instance, along the horizontal plane, the shoulders would be superior, or nearer to the head, than the knees. The knees would be described as inferior, or nearer to the feet, than the shoulders. Similarly along the frontal plane, anterior and posterior refer to parts of the body closer to the ventral and dorsal, or front and back, areas of the body.