When driving your car, you rely on your wing mirrors to give some vision down the side. However, if you look in them when towing your or trailer caravan, you may get a good view of what you are towing, but not a lot else. A blind spot mirror is an attempt to deal with this problem.
The idea is to broaden your view of the area to the rear and the rear quarter, giving a different angle and perspective on the environment around you as you travel.
These types of blind spot mirrors will usually come on a long bracket. These can be fixed on the existing mirror or its bracket and put greater distance between the side of the vehicle and the mirror. This in turn gives the wider view being sought.
If you are going to be doing a lot of towing, you might consider a replacement fitting altogether. The market does include extendable brackets for mirrors. You can then position them in the normal place when just driving your car and then move them, sometimes electronically, out further when towing.
Another alternative is to fix bubble mirrors or frog-eyed mirrors on the existing mirrors. These give a wider view, not because of their physical fixing in a different place, but because of the convex lens, giving a much wider view from the same location.
Some people suggest anyway that convex lenses should be used on the near side to compensate for the particularly large blind spot on that side caused by the distance from the driver. However, they tend to reduce the size of the object you are trying to avoid so they have their down sides as well.
Blind spot mirrors should be checked for secure fitting before travel and adjusted to give the best possible view. The driver will need to be familiar with the new view and more importantly, the area still within the blind spots.