Cadillac 2005 DeVille Owner's Manual page 28

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 pelvic bones. And you would 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 is a sudden stop or a crash,
or if you pull the belt very quickly out of the retractor.
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CAUTION:
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You can be seriously hurt if your shoulder belt
is too loose. In a crash, you would move
forward too much, which could increase injury.
The shoulder belt should fit against your body.
To unlatch the belt, just push the button on the buckle.