Service manual

P
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.
Shoulder Belt Tightness Adjustment (Sedan De Ville)
Your Sedan De Ville has a shoulder belt tightness adjustment feature. If
the shoulder belt seems too tight, you can adjust it like this: