Laue method for determining orientation

[afrano]

I have just seen a photograph known as a Laue photograph. I am reading about the Laue method of X-ray Diffraction. This method is supposed to be useful for determining the orientation of a crystal, according to Ashcroft. My question is how? Orientation with regards to what? It seems to me the concept of orientation is a bit trivial, unless you define a reference or axis, so how is this method helpful?
Thanks.

 

[nbo10]

The diffraction pattern will allow you to determine the crystalline axis, a,b, and c.

 

[nasu]

You can determine the orientation of the crystal axes relative to the specific sample shape.
You can have a thin silicon disc (wafer) and want to know which crystal direction is perpendicular to the face, for example. It can be 100 or 111 etc.