GMC Sierra Denali Owner's Manual page 11

Reclining Seatbacks
Your vehicles front seatbacks have a recline feature.
Vertical Control: You can use the vertical control to
adjust the angle of the seatback. Move the reclining front
seatback rearward or forward by moving the vertical
control toward the rear or toward the front of the vehicle.
But don't have a seatback reclined if your vehicle is
moving.
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