GMC Sierra 2000 Owner's Manual page 51

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A rear facing infant restraint (B) positions an infant
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to face the rear of the vehicle. Rear facing infant
restraints are designed for infants of up to about
20 lbs. (9 kg) and about one year of age. This type
of restraint faces the rear so that the infant's head,
neck and body can have the support they need
in a frontal crash. Some infant seats come in two
--
parts the base stays secured in the vehicle and
the seat part is removable.
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