If you wanted to describe the location of your liver, then one way to do that would be to use sagittal planes. The midsagittal and sagittal planes serve as maps to the human body and its parts. Locations of organs and other body parts are often illustrated by referencing particular sagittal planes. The midsagittal plane is the basis on which other sagittal planes are calculated.
Midsagittal Planes
The fictitious midsagittal plane, also known as the median plane or midline, runs vertically through the middle of the human body, dividing it into two equal left and right segments. Visualize it as drawing a crayon line from the center of your forehead down between your eyes, bisecting your nose and mouth, and continuing the length of your body to the ground between your feet. Remember, though, this is a plane, not a line, so it passes through your body, coming out the front and the back. Think of it like the earth's international date line.
Sagittal Planes
Sagittal planes are imaginary vertical planes passing through the human body along the y-axis, parallel to the midsagittal plane. They divide the body into left and right parts of unequal size. These make-believe lines are often used to explain the placements of various body parts. For example, a sagittal plane could be constructed passing through your left eye. It not only denotes the position of your eye, but also the position of your left internal organs relative to that plane.
Organs and Other Body Parts
Midsagittal and sagittal planes can also refer to the imaginary division of body parts such as the brain. MRIs produce 3-D images of the body's soft tissues. The process can "divide" or "section" the brain along sagittal planes to obtain detailed views of the organ for study and diagnostic evaluation. The term "sagittal plane" is often used in place of the phrase "y-axis" when describing the brain.
Considerations
Only one midsagittal plane exists. Think of it as the prime meridian: there can be only one. There can be numerous sagittal planes, however, as they can be placed anywhere on the body or a body part for descriptive purposes, as long as they parallel the midsagittal plane -- just as global longitudinal lines parallel the prime meridian.