GMC 2006 Sierra 1500 Pickup Owner's Manual page 163

Adjust the mirror by turning it left or right. Once the mirror
is adjusted to the correct position, tighten the set screw to
hold the mirror in place. These mirrors can also be
manually folded in.
On the lower portion of the mirror is a convex mirror.
A convex mirror's surface is curved so you can see more
from the driver's seat.
If your vehicle is equipped
with this style camper-type
mirrors, they can be
adjusted so you can have
a clear view of the objects
behind you.
If your vehicle is equipped with this trailer towing type
mirror, they can be adjusted so you can have a clear view
of the objects behind you. Pull out the mirror head to
extend it for better visibility when towing a trailer.
These mirrors can also be manually folded in. On the
lower portion of the mirror is a convex mirror. A convex
mirror's surface is curved so you can see more from
the driver's seat.
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