When sighting (with a gun for instance), you can easily determine with which eye you aim by making a circle with your thumb and finger and holding at arm’s length away from you. Then with both eyes open, look at a distant object through the circle. Without moving your hand first close one eye and then the other. Whichever eye keeps the object within the circle is your dominant eye and should be used for sighting. (Most people are right eye dominant.) – Rina
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