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?

  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"?

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    $\begingroup$ 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. $\endgroup$ – MaxW Apr 19 '16 at 16:22
  • $\begingroup$ 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. $\endgroup$ – Karl Apr 19 '16 at 18:13
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    $\begingroup$ 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. $\endgroup$ – Ivan Neretin Apr 19 '16 at 18:17
  • $\begingroup$ We're getting somewhere, I think. "n-gonal" means "one Cn axis, angles are 90°, resp. fixed by the symmetry" $\endgroup$ – Karl Apr 20 '16 at 20:03
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    $\begingroup$ "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. $\endgroup$ – Oscar Lanzi Apr 23 '16 at 14:14

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