Sorry if this is the wrong forum, but I figure it is the most relevant since it deal with crystal structures. I'm taking a materials science course currently, and have some confusion on the definition of close packing. Since it seems to represent different things depending on if it is viewed linearly, planar, or with volume. Would someone just please clarify that I have these bits thought out correctly? Volume: Close packed in volume refers to the maximum amount of nearest neighbors, which is 12. If it doesn't have CN=12, it is not close packed. There is also no "closest packed" structure we can view within a unit cell. It's either close packed or not. Planes: Close packed planes will have atoms with CN=6. But there are also what are referred to as closest packed planes. Which is the plane in the unit cell with the highest CN, although it may not be equal to 6. For example, the body centered cubic has the closest packed plane on (1,0,1), with CN=4. It is the closest packed plane in that structure, but not a close packed plane. Lines: Close packed lines are the directions that can be drawn in a unit cell that has the closest packing. Which basically means it has CN=2 on the line. There isn't a closest packed line, since there is really only 2 bonds on any given line. That's about it. I want to make sure I have all the terminology right, and I don't have any misconceptions with it. I get the whole idea of packing, my teacher just never clarified the vocab.