GMC 1996 Sierra Owner's Manual page 66

Door Locks
There are several ways to lock and .unlock your vehicle.
I
Unlocked doors can be dangerous.
Passengers
--
especially children
--
can easily
open the doors and fall out. When a door is
locked, the inside handle won't open it.
Outsiders can easily enter through an unlocked
door when you slow down or stop your vehicle.
This may not be so obvious: You increase the
chance of being thrown out of the vehicle in a
crash if the doors aren't locked. Wear safety belts
properly, lock your doors, and you will be far
better off whenever you drive your vehicle.
From the
outside, use your key or the Keyless Entry
System, if.your vehicle has this option.
To lock the door from the
inside, slide the lever on
your door down.
To unlock the door, slide the lever up.
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