The primary purpose of proper posture in corps-style marching is to present the performer as a confident, aggressive individual. This is accomplished by making every element of the body, from the feet to the head, appear as tall and big and strong as possible, and relaxing muscles to eliminate all appearance of tension.
For wind players, posture also serves the secondary purpose of eliminating constrictions in the airstream from the bottom of the lungs to the mouth. This improves dynamic flexibility, as well as tone quality and the breath support that is needed to maintain it.
All other elements of corps-style marching depend on proper posture to be effective.
Proper distribution of the body's weight along the feet is the most fundamental part of maintaining correct posture. Incorrect weight distribution makes it impossible to maintain upper-body posture while moving (especially at slower tempos) and can create great difficulty in standing still for long periods of time.
With both feet flat on the ground, approximately 70% of the body's weight should be supported by the balls of both feet, and the remaining 30% on the heels. When done properly, in combination with the rest of the elements described in this chapter, this will cause the entire body to be straight up and down.
Most performers, when standing properly, will actually feel as if they are leaning forwards somewhat. This is because most people typically lean back slightly when they stand, so that actually standing perfectly vertical is noticeably "more forwards" than they are used to. Some performers, upon noticing this sensation, will actually force themselves to lean backwards so that they feel like they are standing straight up and down. Others, knowing that they should feel as if they are leaning forwards, will actually introduce a deliberate forward lean into their posture (usually at the waist, although sometimes at the ankles), causing them to actually be leaning forwards. Although both of these tendencies are trivial for an instructor, who can see the performer's entire body, to notice and correct, the performer himself can notice and correct these flaws by paying attention to the distribution of weight on his feet.
Proper weight distribution, in addition to being the most fundamental part of keeping the entire body straight, is also essential for proper fluid movement. As will be explained later, with every step the performer takes the entire weight of the body must be shifted from one foot to the other. By putting most of the weight towards the front of his foot, the performer is better able to control his momentum and therefore both the direction and timing of his steps.
When standing still, feet are generally placed either alongside each other, parallel and pointing straight ahead, or with ankles together, toes pointed 45 degrees apart (22.5 degrees on either side of straight ahead). The first method is known generally as "feet-forward", while the second is typically referred to as "feet-45". Feet-45 is widely considered to be more aesthetically pleasing than feet-forward, and is also a way of paying homage to marching music's military roots, but is considerably more difficult to clean. While feet-forward is trivial to teach and understand, feet-45 relies on the performer's muscle memory to understand, which requires that it be taught properly in the first place.
To ensure similar angles throughout the ensemble, groups that use feet-45 typically have guides made of wood or heavy cardboard that they can place between the performer's feet to set them at the correct angle. The performer lines his feet up along both arms of the footguide and memorizes the feeling in his ankles and upper thigh, and then recreates that feeling whenever he is standing with the ensemble.