The side impact test is carried out by firing a deformable barrier at 30mph into the driver's side of a stationary car.
Under these circumstances, the driver's head is the most frequently seriously injured part of the body.
The introduction of side impact airbags prompted substantial safety gains, resulting in Euro NCAP introducing a pole test as a more severe test of head protection.
The pole test is carried out by firing the driver's side of the car into a rigid, narrow pole at 18mph. Because of the narrowness of the pole, the impact forces are concentrated and result in a more severe intrusion into the side of the car.
What Euro NCAP looks out for
This test replicates a car hitting a roadside light or sign, and measures the effectiveness of side impact airbags, particularly in stopping the driver's head penetrating the side window and hitting the pole.
While the strength of the car is important, the way in which the side airbags deploy is much more significant in protecting the car's occupants.