Chevrolet 2008 Silverado 1500 Pickup Owner's Manual page 36

3. Be sure that the belt is not twisted and it lies flat.
The elastic cord must be under the belt and the
guide on top.
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CAUTION:
{
A safety belt that is not properly worn may not
provide the protection needed in a crash. The
person wearing the belt could be seriously
injured. 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.