Symmetry Groups, of which Frieze groups are a subset, are spoken about using certain words. This second post of intro (here’s the first) will begin to explain the words that are used to describe the seven frieze groups. There are only 5 terms to learn. There’s any number of places where these are written out and described. I have nothing new to add, but identifying the words that describe the way symmetries grow seems like the only way to start.
In future posts I’ll draw back the curtain on these terms in ways that will make them seem richer than their definitions, and less confusing than their examples.
Here are the five terms that describe the ways that frieze groups grow:
- vertical reflection
- horizontal reflection
- glide reflection
- 180 degree rotation
Translation simply means repeating something without any changes. If I write the letter “I” like this “IIIIIIIII” that’s translation.
Vertical reflection reflects a shape across a vertical line. A “W” can be seen as a vertical reflection of a “V.” This means when the shape is vertically reflected, the pattern grows horizontally. Sorry. That’s just how it is.
Horizontal reflection reflects a shape across a horizontal axis, like tree reflecting in water. Which means that when a shape is horizontally reflected, the first action of this pattern grows it vertically. This takes some getting used it.
Glide reflection makes a copy of the original unit, slides it over, then horizontally reflects it. There’s no need to try to wrap you mind around this one until you see examples.
180 degree rotation is a half of a full rotation. If you are facing one way and you turn around to face what was behind you, that’s a 180 degree rotation. There’s more to know about this one, so much more that, before I getting to examples, I will writing yet one more introductory post focussing on 180 degree rotations. If you don’t understand how a 180 degree rotation can happen around different points, you will soon be lost, so read the next post.
Each of the seven symmetry groups contain translations, which is to say there is repetition of some or no variation of the original tile. The rest of the frieze symmetry groups are made up of combinations of the rest of the ways frieze symmetries grow.
There are many different systems of notations around symmetry groups.
I’ve put the main ones I’ve been seeing together on the page above, though there are more notational systems then just these. For awhile it drove me nuts not knowing how each of these notations corresponded to each other.
The first column has F’s, which I am just going to assume stand for frieze. There is nothing inherently descriptive about this notation, but it is kind of nice to be able to put the groups in a numerical order.
The second column is the Hermann–Mauguin notation (or IUC notation),. The “p” stands for plane, “g” is glide reflection, the 1’s are placeholders, and sometimes aren’t written out, “m” references mirroring, and I’m not sure what the 2 means. I did know but I forgot. I’ll make an edit when I figure that out again.
The third column appears to be the most scholarly method, call Orbifold notations. I wrote to Alex Berke, whose book Beautiful Symmetries will be hot off the MIT presses in March 2020, to demystify this notation for me. This was part of the response I got back:
Each symbol corresponds to a distinct transformation:
- an integer n to the left of an asterisk indicates a rotation of order n around a gyration point
- an integer n to the right of an asterisk indicates a transformation of order 2n which rotates around a kaleidoscopic point and reflects through a line (or plane)
- an indicates a glide reflection
- the symbol indicates infinite rotational symmetry around a line; it can only occur for bold face groups. By abuse of language, we might say that such a group is a subgroup of symmetries of the Euclidean plane with only one independent translation; the frieze groups occur in this way.
- the exceptional symbol o indicates that there are precisely two linearly independent translations.