# Miller indices: How does one translates the orientation to the origin of the unit cell?

In a section discussing Miller indices, my textbook says the following:

The rules for determining the Miller indices of a direction or an orientation in a crystal are as follows: translate the orientation to the origin of the unit cell, and take the normalized coordinates of its other vertex.

What I'm confused about is how one translates the orientation to the origin of the unit cell?

Since the textbook just mentions this without elaboration or illustration, it is not clear precisely what this means or how one would go about it. I would appreciate it if people could please take the time to explain and demonstrate how this is (mathematically) done. Sources would be appreciated, if you have them.

• When you reach a confusing point like this, consult another book or source. I usually found it beneficial to read the same material from 2-3 books. Multiple sources gave me a better understanding of the topic being studied.
– MaxW
Jul 17, 2019 at 15:53
• @MaxW I attempted to find clarification using a Google search before I posted this question. See the slide beginning Determine the Miller indices for the plane: www3.nd.edu/~amoukasi/CBE30361/Lecture__crystallography_B.pdf It's the same problem: The author mentions the process, but I don't see any explanation or illustration of how it is actually done. Jul 17, 2019 at 16:01
– MaxW
Jul 17, 2019 at 16:23
• @MaxW I did, but there isn't even any mention of translations or "origin". Jul 17, 2019 at 16:24
• @ThePointer Note that you may train yourself a bit, e.g. on doitpoms.ac.uk/tlplib/miller_indices/index.php Jul 17, 2019 at 20:53

$$P_2' = P_2 - P_1$$
$$P_1' = P_1 - P_1 = (0,0,0)$$
Again, the pair of two points still points in the same direction, but now $$P_1$$ is at the origin. Taking the fractional coordinates (i.e. along the unit cell axes) of $$P_2'$$, you can turn those into Miller indices $$h$$, $$k$$, and $$l$$ by multiplying with a large number and rounding to the next integer. Generally, the direction will be off because of the rounding. However, in special cases, e.g. if your DNA is on some crystallographic symmetry axis, the Miller indices will match your direction exactly.