Buick Lucerne 2007 Owner's Manual page 56

Lower Anchors
Lower anchors (A) are metal bars built into the
vehicle. There are two lower anchors for each
LATCH seating position that will accommodate a
child restraint with lower attachments (B).
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Top Tether Anchor
A top tether (A, C) anchors the top of the child
restraint to the vehicle. A top tether anchor is built
into the vehicle. The top tether attachment (B)
on the child restraint connects to the top tether
anchor in the vehicle in order to reduce the forward
movement and rotation of the child restraint
during driving or in a crash.
Your child restraint may have a single tether (A) or
a dual tether (C). Either will have a single
attachment (B) to secure the top tether to the
anchor.