Child car seat manufacturers are bound by very strict guidelines, and you may be surprised to find that some of the more expensive car seats don't provide any better level of protection than those which are more moderately priced. Many State laws require children to be in a safety car seat only up to age 4 or so, and many parents assume that children who are older than this will be safely restrained by just an adult safety belt.
This, however, is not the case. An automobile's seat belt system is designed for use by adults, not small children, and it usually will not fit a child properly. A booster car seat is designed to improve the fit of the vehicle's safety belt system across the child's body, for greater comfort and safety.
High back car seats should always be used in cars without headrests, or with low seats, both to provide the child with a comfortable place to rest his or her head, and to provide protection against "whiplash" in an accident. Low back car seats are generally cheaper than high back car seats, and are easier to move from car to car, but should only be used in cars with headrests and high seat backs. High back car seats are often available with a 5 point harness (when car seats are not being used with a harness, they are considered being used in the "belt positioning" mode).
A booster car seat are designed to raise the child up so that the lap and shoulder seat belts fit properly. Belt positioning car seats must always be used with a car's lap and shoulder belt (never with a lap only belt). The car seat will raise a child so the lap belt goes over the child's pelvis and hips (instead of his or her abdomen), and the shoulder belt crosses the middle of the child's chest and shoulder (the car seat may have a clip that can be adjusted to ensure that the shoulder belt does not cross the child's face or neck).
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