Cadillac 2004 DeVille Owner's Manual page 20

4. Push the latch plate into the buckle until it clicks.
Be sure to use the correct buckle when buckling
your lap-shoulder belt. If you find that the latch plate
will not go fully into the buckle, see if you are
using the buckle for the center passenger position.
Pull up on the latch plate to make sure it is
secure. If the belt isn't long enough, see Safety Belt
Extender on page 1-27.
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.
1-14
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 crash,
or if you pull the safety belt very quickly out of the
retractor.