Blindspots are areas which are not covered by your peripheral vision or your mirrors. It is surprisingly easy for another road user to disappear in one of these blindspots. It is also surprising how close to you they can get while in this hidden position.
With effective and regular use of mirrors it will be very difficult for another road user to get into a blindspot without you knowing about it. However, anytime you find yourself in a situation in which you are not sure what is there, a quick glance over your shoulder is all that is needed to fill in the gap.
It is also important to be aware that additional blindspots are created by the pillars which connect the roof of a car to its body. Most cars have two pillars at the front (either side of the windscreen), two in the middle, and two more at the back (either side of the rear window). It can be surprising how easily another road user can disappear into one of these blindspots. Be particularly mindful of the possibility of cars, motorcyclists and cyclists being hidden in this way when you are emerging at junctions. When reversing, watch out for pedestrians who may be hidden by the rear pillars. Move your head around to ensure you are checking these blindspots whenever necessary.