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Seven pieces of baby car seat safety jargon you need to know before you buy

Get the lowdown on i-size, isofix, belt guides, travel systems and more

Seven pieces of baby car seat safety jargon you need to know before you buy

Baby car seats are an essential piece of child safety equipment, but the terminology used can be unfamiliar and confusing.

Before shopping for a new car seat it’s key you get to grips what can seem confusing jargon to help ensure you choose the most appropriate car seat for your little one.

Also see our round up of the best car seats that most impressed in our rigorous car seat crash tests.


1. i-Size

i-Size is the newer European child car seat regulation (ECE R129). It was introduced in 2013 and came into force in the UK in April 2015.

There are several notable differences between R44.04 (the older regulation) and R129, which you can see in the table below.

ECE R44.04

ECE R129 (i-Size)

Based on a child’s weight (kg)

Based on child’s height (cm)

Installed either using Isofix or adult seat belt

Installed only using Isofix

Can turn forward-facing from 9kg (approximately nine months)

Rearward-facing until at least 15 months old

Only rear and front impact tests are mandatory

Rear, front and side impact tests mandatory

As well as being easier to install and rear-facing for longer, which is safer for baby, i-Size car seats also offer better car compatibility.

All baby and child car seats with an i-Size logo should fit in all i-Size-approved cars – look for the i-Size logo to see whether it’s i-Size compatible.

i-Size car seats undergo more stringent crash testing compared to R44.04 child car seats. But car seats approved to R44.04 are still perfectly legal to buy and use.

Our child car seat tests are similar to R129 as they include a side-impact crash test. We put all car seats (including R44.04 seats) through our full tests.

We’ve found 20 Best Buy i-Size car seats and 3 Don’t Buys to avoid.

2. Isofix

Isofix was designed to make clicking a child car seat into your car more easy. This should reduce the risk of it being installed incorrectly, thereby reducing the chances of your car seat failing in a crash because of dodgy installation.

Almost all new cars built since 2002 should have Isofix connectors fitted as standard.

Isofix in your car can be identified by two slots beside each rear seat. Some slots are easy to find and quite accessible, but others can be a bit hidden – check your car manual if you’re unsure.

The two metal connectors on Isofix child car seats fit into these slots and directly attach your car seat to the chassis of your car.

Although Isofix makes installing your car seat quick and easy, in our tests we sometimes see differences in crash test results depending on whether a child car seat is installed using Isofix or the adult seat belt.

Go to our child car seat reviews to see how each installation method compares in terms of the crash test star ratings.

3. Support leg

Rear-facing Isofix baby child car seats are generally suitable for children up to four years old.

This type of car seat needs three anchor points in your car, so you’ll need to use a top tether (explanation below) or a support leg, as well as the two Isofix points.

A support leg extends from the base of your child’s car seat to the floor of the car and prevents the seat from tipping forwards in a collision.

Support legs are more commonly found on child car seats than a top tether, and in our tests they are easier to install and use.

But if your car has underfloor storage compartments in the rear, you may not be able to use a car seat that has a support leg and will need a top tether instead.

Visit our best rear-facing baby car seats guide to see the top-scoring car seats with a support leg, as well as the ones that performed poorly in our tough crash tests.

4. Top tether

A top tether is a fabric strap that extends from your car seat and fixes to an anchor point in your car that’s usually somewhere behind your car’s back seat or near the boot.

Similar to a support leg, it’s the third fixing point and it prevents your rear-facing child car seat from rotating forwards in a crash.

When securing the top tether, it’s vital to make sure you’re attaching the strap to the correct anchor point in your car.

Sometimes a headrest stem or luggage tether fixing can be mistaken for a tether point, but neither of these is designed to withstand the forces of a crash and could put your child at risk if you use them.

If you can’t find the top tether mount to secure your rear-facing child car seat, refer to your car’s manual to find out where it is in your car or contact the car manufacturer.

Go to our 10 quick child car seat checks for advice on fitting your car seat, including safely installing child car seats that have a top tether or support leg.

5. Belt guides

Belt guides are coloured indicators on each car seat to help you install it correctly when using the car’s adult seat belt to attach it to your car.

It’s vital you get the belt routing correct, as the belt guiding is designed to secure the seat and direct the force of a crash away from your child’s body, to reduce the risk of injury.

There are two different types, depending on what stage of seat you are fitting:

  • Blue belt guides are on rearward-facing car seats that can be installed using the car’s adult seatbelt.
  • Red belt guides are on forward-facing car seats to show you where the adult seat belt should be routed to hold the child and the car seat in place correctly.

With forward-facing seats there is usually one indicator near the headrest to position the diagonal belt across the shoulder properly, to stop the seat belt digging into your child’s neck.

Clear instructions to help you install the car seat must be printed on the side of each baby or child car seat, and we’d always recommend using this as well as watching any manufacturer videos on how to install your car seat correctly.

For more information on getting the best car seat, go to our guide on how to buy the right baby or child car seat.

6. One-pull harness

Getting a wriggly child into a child car seat harness and tightening the straps properly can be a challenge, but a one-pull harness makes it much easier.

This is because it has a strap that allows you to pull both shoulders of the harness tight at the same time.

A correctly fitted harness is vital to help keep your little one as safe as possible in a crash.

Make sure to remove any thick clothes before putting your baby or child in their car seat, as bulky clothing can make the harness less effective.

The harness buckle should not be over your child’s tummy and the harness itself should be quite tight, so that only one or two fingers can fit between your little one’s chest and the harness strap.

Make sure you’re fitting your baby or child car seat correctly, see our guide on fitting a baby or child car seat.

7. Travel system-compatible car seats

Some baby car seats can be popped onto the chassis of a pushchair, usually with adaptors. This arrangement is called a travel system.

It means that you can easily transfer your child from pushchair to car without disturbing them too much.

Many leading brands of child car seats, such as Britax, Joie or Maxi-Cosi, can be used with a wide range of pushchairs.

The car seat will either:

  • attach to the chassis instead of the pushchair’s seat – common on pushchairs with reversible seat units.
  • click on over the existing pushchair’s seat – common with pushchairs that have forward-facing seats

A baby under four weeks old should not be in a car seat for longer than 30 minutes, and ideally for no longer than necessary as their body is scrunched up which can affect their breathing and put strain on their developing spine.

Older babies shouldn’t be in a baby car seat for longer than two hours, so if you plan to go on a shopping trip or a long walk, use a lie-flat carrycot with your pushchair instead.

Take a look at our guide to choosing the best travel system for our top 10 best travel systems.

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