GMC 2002 Envoy - Manual (Page 55)

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Top Strap
Some child restraints have a top strap, or “top tether.”
It can help restrain the child restraint during a collision.
For it to work, a top strap must be properly anchored to
the vehicle.
If the child restraint does not have a top strap, one can
be obtained, in kit form, for many child restraints.
Ask the child restraint manufacturer whether or not a kit
is available.
In Canada, the law requires that forward - facing child
restraints have a top strap, and that the strap be
anchored. In the United States, some child restraints also
have a top strap. If your child restraint has a top strap, it
should be anchored.
Anchor the top strap to one of the following anchor
points. Be sure to use an anchor point located on the
same side of the vehicle as the seating position where
the child restraint will be placed. Raise the head restraint
and route the top strap under it.
Once you have the top strap anchored, you’ll be ready to
secure the child restraint itself. Tighten the top strap
when and as the child restraint manufacturer’s
instructions say.