The origin of this interesting bit of art probably occured when someone, while shooting film, forgot to forward the roll to the next empty slot. As they shot, the already exposed negative got re-exposed and the photos superimposed creating something interesting and Voila! double exposure is born.
Now a days, we dont need to fiddle with the films, maybe just a bit in the digital darkrooms to get ourselves a nice double exposure photo. Some cameras and apps on phones allow you to take amazing double exposure shots.
Go ahead and make wonderful surreal photos and share them away!
Last time, D was for Depth of Field.