- We name the joints at which movement occurs
- We describe the motion at ONE JOINT AT A TIME.
- At each joint, we describe the motion in ONE PLANE AT A TIME

- Sagittal
- Frontal (sometimes called the coronal plane)
- Transverse (sometimes called the horizontal plane)

Another view of the cardinal planes

When we describe motion, we mentally *move* the plane so that it "cuts through" the joint at or near the joint surface. Imagining a plane which cuts through at the level of a joint permits us to draw a two-dimensional (or planar picture of the joint:

Each plane is defined by an axis, a line that is perpendicular to the plane. When we observe a drawing of the joint in a single plane, we appreciate the joint's axis as a point; in the drawings linked above, an 'x' marks the joint axis.