Symmetric procedural material I've modelled a shape which is symmetric along z axis. I used material and some texure with generated coordinates plugged in displacement node. Everything is distributed evenly on the mesh. 
I would like that material is also mirror image along z.

So that a procedural texture on one side of z axis 
is the mirror image of other side of z axis.

How to achieve this with nodes?
Mix Color

First step is to mask out the surface's point that be affected by the flipping of the coordinates.

In order to get the reference needed we should start by adding a Texture coordinates node and pick the Object coordinates
   type (if you are interest in the local Z)

Then add a Separate XYZ to get the Z component of the coordinates and filter out the values greater than 0 with the help of a Math node



If the origin of the object belongs to the simmetry plane, all we have to do is to flip the input coordinates (Generate type in the shown example) with a Mapping node (notice the -1 value for the Z scale parameter) and mix them with a Mix node whose inpuct factor is the mask we created before.