# How to make sense of the crystal systems names

I cannot fix the names and structures of the seven crystal systems in my head. There's a thousand books explaining their geometry and it's totally fine and understandable, no problem. But what do the names mean, etymologically?

1. Cubic
2. Tetragonal
3. Orthorhombic
4. Monoclinic
5. Hexagonal
6. Rhombohedral or Trigonal
7. Triclinic

OK, cubic is simple enough, hexagonal too. I know that "-eder/-hedral" means "side" and "-gon" means "angle". Why is a system with three different edges and angles triclinic? What are the rationales behind each of the names?

A rhombus has sides of equal length. Orthorhombic has only right angles (orthogonal), but the sides a,b,c have different lenghts. So why "rhombic"?

• If you're looking for one cohesive system of nomenclature, then there isn't one. If you want the etymology of each word then look the words up in a dictionary. – MaxW Apr 19 '16 at 16:22
• I want the etymology (thanks for the word ;)). Sadly, the dictionaries (merriam-webster.com/dictionary/triclinic ,oxforddictionaries.com/de/definition/englisch/orthorhombic) are not helpful. They just repeat the definition. – Karl Apr 19 '16 at 18:13
• AFAIK, "-clin" is a cognate of "inclined", as in "not straight", so triclinic and monoclinic both make perfect sense: they have three and one non-$90^\circ$ angles, correspondingly. – Ivan Neretin Apr 19 '16 at 18:17
• We're getting somewhere, I think. "n-gonal" means "one Cn axis, angles are 90°, resp. fixed by the symmetry" – Karl Apr 20 '16 at 20:03
• "Trigonal" and "rhombohedral" come from different features of that system. The former comes from the presence of a threefold rotation axis (the principal unit-cell axes are distributed around this threefold axis, so they have the same lengths and angles between them). "Rhombohedral" comes from a polyhedron with this ($D_{3d}$) symmetry whose faces are all rhombi. – Oscar Lanzi Apr 23 '16 at 14:14

Monoclinic and triclinic

Ivan (in the comments) is spot on about these two. Among all the systems, these are the only ones with angles that are not fixed by symmetry, so you actually have to measure them.

Rhombohedral or Trigonal

These are two different settings of the same lattice. The unit cell can either be a prism (with angles of 90, 90 and 120 degrees, blue lines in the figure) or a rhombus (with all three angles equal, and all three sides equal, black lines in the figure). In general, there are infinite choices for unit cells (you just need three translation vectors that are not linearly dependent, so pick an origin on a lattice point and three other lattice points and its a unit cell). The conventional choices usually have more of the symmetry elements along the axes, face diagonal or body diagonal, and keep the volume of the unit cell fairly small.

Orthorhombic

This used to be called pseudo-orthorhombic. Again, you have a choice of unit cell, and the more uncommon one is a rhombohedral prism. When you switch from all right angles to a rhobohdral prism setting, face-centered turns into primitive and primitive turns into face-centered.

Here is a quote from Wikipedia:

In the orthorhombic system there is a rarely used second choice of crystal axes that results in a unit cell with the shape of a right rhombic prism;2 it can be constructed because the rectangular two-dimensional base layer can also be described with rhombic axes. In this axis setting, the primitive and base-centered lattices swap in centering type, while the same thing happens with the body-centered and face-centered lattices.

Historically, the first data available to crystallographers (and minerologists) were the shapes of crystals. Orthorhombic crystals can have faces that are at 90 degree angles or not, depending on the kinetics of growth. Pictured below are three crystal forms with underlying orthorhombic symmetry, with the 2nd one showing is a shape with (+/- 1, +/- 1, +/-1) faces labeled orthorhombic pyramid: Cubic, Hexagonal and Tetragonal