Dendrites are formed on the surfaces of a rock. However, as the rock is covered with more rock, there becomes more potential surfaces.
We don’t know exactly how it happens, but when the Manganese seeps into cracks in the rock, it coats both surfaces fairly evenly. When you get both sides of a rock that has broken, you will have two images mirroring each other. This is called a Double.
Sometimes you get the complete picture, mirrored on both sides, sometimes one side is sharper than the other, sometimes, you get just a fragment of the double, as the rest of the rock has broken off.
And sometimes you get a double, with another double on the backside of one rock, a triple (or quadruple or more.) (below)
And this can go on several times.
A double almost always increases the value of a good dendrite by two or three times, and can occasionally make a poor dendrite into a good one.
A Triple is just that. A stone breaks into 3 pieces, and you get mirror images in two pairs. The middle stone will have an image on both sides that matches on or the other two stones. A Bonus is when the back of a double triple or puzzle also has a good picture on one or more of the non-mirrored sides.
A Set is a collection of similarly pictured rocks, usually broken off one larger rock.