GMC Sierra Owner's Manual page 58

Sit up straight and always keep your feet
on the floor in front of you (if possible).
Always use the correct buckle for your
seating position.
Wear the lap part of the belt low and
snug on the hips, just touching the
thighs. In a crash, this applies force to the
strong pelvic bones and you would be
less likely to slide under the lap belt.
If you slid under it, the belt would apply
force on your abdomen. This could cause
serious or even fatal injuries.
Wear the shoulder belt over the shoulder
and across the chest. These parts of the
body are best able to take belt
restraining forces. The shoulder belt locks
if there is a sudden stop or crash.
Warning
You can be seriously injured, or even
killed, by not wearing your seat belt
properly.
Never allow the lap or shoulder belt to
become loose or twisted.
Seats and Restraints
Never wear the shoulder belt under both
arms or behind your back.
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