Graco Booster Seat Owner's Manual page 17

When using back support, the child's ears are
below top of booster seat. If tops of ears are
above top of seat, your child is too large for the
booster seat.
Without back support, when sitting on the booster
seat, your child's ears are below the top of the
vehicle seat cushion / head rest.
Your Graco booster seat can be used with or without
the back support (on certain models) as long as the
requirements on page 16 are met.
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