Every car seat has a harness, which is the pieces of webbing and the buckles that actually hold the baby in the seat. In order to make the harness fit the baby properly, you have to change the harness height and tighten or loosen the webbing.
On many car seats, you change the harness height by moving the straps into harness slots at various heights. To move the harness straps, you first have to remove the ends from a splitter plate beneath the car seat.
The splitter plate is a flat piece of metal with hooks or bars on each side. The harness webbing goes through the harness slots and down the back of the car seat, where loops on the ends of the harness webbing slip over the hooks on each end of the splitter plate. Once you've removed the harness webbing ends from the splitter plate, you take the straps out of the harness slots, and then re-thread them into the new set of harness slots before replacing the ends on the splitter plate.
A no-rethread harness means that you don't have to take the strap ends off the splitter plate and put them through the individual slots. You can still change the harness height, though. On no-rethread harness car seats, the harness height is often changed by pulling on levers, pushing buttons, or turning knobs. These devices raise and lower the harness so that it sits at the right place.
Some modern car seats have a harness that can be adjusted to various heights simply by pulling up or pushing down on the webbing straps.
One example of this style is the Evenflo Symphony 3-in-1 car seat, which can be used as a rear-facing car seat, a forward-facing car seat, and a high back booster.
The harness should be at or below baby's shoulders for rear-facing, and at or above baby's shoulders for forward-facing. That rule applies whether you're using a no-rethread harness or one where the harness height must be changed using slots.
Always check the car seat instructions to ensure you're placing the harness correctly.
The car seat shown here, the Chicco NextFit convertible car seat, has a no-rethread harness that is operated by the orange button on top of the car seat shell. When you push the button, the entire head-wing portion of the car seat slides up and down, and the harness height moves up and down, as well.
Some new car seat models have a no-rethread harness that needs to be threaded differently for various modes. An example is the Safety 1st Grow & Go Air 3-in-1 car seat. The rear-facing mode has a very low set of harness slots for newborns. To access this set of harness slots, you have to take the ends of the harness webbing off the splitter plate just like with a traditional harness, then thread them through the lowest slots and replace on the splitter plate. When the baby is ready for a higher set of slots, you move the webbing back up, and the upper slots function as a no-rethread harness system. It's important to read the instructions for your child's car seat to make sure you're using the harness system correctly.
Heather Corley is a certified Child Passenger Safety Technician-Instructor.