Posture is the way people hold their bodies upright against the downward pull of gravity. Proper alignment of the spine is necessary for good posture. The spine is not straight; it has three curves, bending inward at the neck, curving slightly out at the upper back and in again at the lower back.
All About Curves
Correct posture alignment should be seen from the side as an imaginary vertical line from the ears, shoulders, pelvis and hips to the knees and ankles. Allowing these curves to get deeper or flatter throws the muscles and joints out of alignment, ultimately causing back pain and fatigue. The head should sit directly above the torso and not poke out in front of it, while the chin should be level with the floor. The shoulders must be relaxed and back, but not forced. Chests ought to be slightly lifted -- as though pulled toward the ceiling by a string -- but also not forced. The pelvis should be relaxed, not tilted either forward or back.