GMC 2003 C4C042 Owner's Manual page 46

The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this applies
force to the strong pelvic bones. And you'd be less
likely to slide under the lap belt. If you slid under it, the
belt would apply force at your abdomen. This could
cause serious or even fatal injuries. The shoulder belt
should go over the shoulder and across the chest.
These parts of the body are best able to take belt
restraining forces.
The safety belt locks if there's a sudden stop or a crash,
or if you pull the belt very quickly out of the retractor.
Each position next to the windows has a shoulder belt
height adjuster. Move the shoulder belt adjuster to the
height that is right for you. Adjust the height so that the
shoulder portion of the belt is centered on your shoulder.
The belt should be away from your face and neck, but
not falling off your shoulder.
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