Buick Century 2003 Owner's Manual page 31

When the shoulder belt is pulled out all the way, it
will lock. If it does, let it go back all the way
and start again. If the belt is not long enough, see
Safety Belt Extender on page 1-29 .
Make sure the release button on the buckle is
positioned so you would be able to unbuckle the
safety belt quickly if you ever had to.
3. To make the lap part tight, pull down on the buckle
end of the belt as you pull up on the shoulder part.
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.
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