GMC 2007 Sierra 1500 Pickup Owner's Manual page 184

Adjust each 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.
184
If your vehicle is
equipped with this style
trailering-type mirror,
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.