GMC 2001 Sierra - Manual (Page 19)

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Reclining Seatbacks
To adjust the front seatback, lift the lever upward.
The lever is located on the outboard side of the seat
cushion. Release the lever to lock the seatback where
you want it. Lift the lever again upward and the seatback
will go to an upright position.
If your vehicle has power seats with a power recliner,
see “Power Seats” in the Index for further information
on how to operate the reclining seatback feature.
But don’t have a seatback reclined if your vehicle
is moving.