The K-9 Vent Lock is the ideal tool to allow your pet the fresh air and ventilation they need in your car. It works by providing a secure link between the boot lid and the boot lock. It also works on rear door and frame on 4 wheel drive vehicles. They work best if you can open the sunroof or one side window a little bit as this will allow warm air to rise up out of the car while drawing in fresh cooler air through the boot. They are available in four sizes, small 2in (50mm), medium 5in (125mm), large 8in (200mm) and extra large 12in (300mm).