GMC Sierra Classic 2007 Owner's Manual page 182

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. The convex
mirror can also be manually adjusted to provide
better vision for the driver.
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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. The convex mirror can
also be manually adjusted to provide better vision
for the driver.