A frieze group is a mathematical concept to classify designs on two-dimensional surfaces which are repetitive in one direction, based on the symmetries in the pattern. Such patterns occur frequently in architecture and decorative art. The mathematical study of such patterns reveals that exactly 7 different types of patterns can occur.
Frieze groups are two-dimensional line groups, from having only one direction of repeat, and they are related to the more complex wallpaper groups, which classify patterns that are repetitive in two directions.
As with wallpaper groups, a frieze group is often visualised by a simple periodic pattern in the category concerned.
Formally, a frieze group is a class of infinite discrete symmetry groups for patterns on a strip (infinitely wide rectangle), hence a class of groups of isometries of the plane, or of a strip. There are seven different frieze groups. The actual symmetry groups within a frieze group are characterized by the smallest translation distance, and, for the frieze groups 4–7, by a shifting parameter. In the case of symmetry groups in the plane, additional parameters are the direction of the translation vector, and, for the frieze groups 2, 3, 5, 6, and 7, the positioning perpendicular to the translation vector. Thus there are two degrees of freedom for group 1, three for groups 2, 3, and 4, and four for groups 5, 6, and 7. Many authors present the frieze groups in a different order.
A symmetry group of a frieze group necessarily contains translations and may contain glide reflections. Other possible group elements are reflections along the long axis of the strip, reflections along the narrow axis of the strip and 180° rotations. For two of the seven frieze groups (numbers 1 and 2 below) the symmetry groups are singly generated, for four (numbers 3–6) they have a pair of generators, and for number 7 the symmetry groups require three generators.
A symmetry group in frieze group 1, 3, 4, or 5 is a subgroup of a symmetry group in the last frieze group with the same translational distance. A symmetry group in frieze group 2 or 6 is a subgroup of a symmetry group in the last frieze group with half the translational distance. This last frieze group contains the symmetry groups of the simplest periodic patterns in the strip (or the plane), a row of dots. Any transformation of the plane leaving this pattern invariant can be decomposed into a translation, (x,y) → (n+x,y), optionally followed by a reflection in either the horizontal axis, (x,y) → (x,−y), or the vertical axis, (x,y) → (−x,y), provided that this axis is chosen through or midway between two dots, or a rotation by 180°, (x,y) → (−x,−y) (ditto). Therefore, in a way, this frieze group contains the "largest" symmetry groups, which consist of all such transformations.
The inclusion of the discrete condition is to exclude the group containing all translations, and groups containing arbitrarily small translations (e.g. the group of horizontal translations by rational distances). Even apart from scaling and shifting, there are infinitely many cases, e.g. by considering rational numbers of which the denominators are powers of a given prime number.
The inclusion of the infinite condition is to exclude groups that have no translations:
- the group with the identity only (isomorphic to C1, the trivial group of order 1).
- the group consisting of the identity and reflection in the horizontal axis (isomorphic to C2, the cyclic group of order 2).
- the groups each consisting of the identity and reflection in a vertical axis (ditto)
- the groups each consisting of the identity and 180° rotation about a point on the horizontal axis (ditto)
- the groups each consisting of the identity, reflection in a vertical axis, reflection in the horizontal axis, and 180° rotation about the point of intersection (isomorphic to the Klein four-group)
Descriptions of the seven frieze groups
There are seven distinct subgroups (up to scaling and shifting of patterns) in the discrete frieze group generated by a translation, reflection (along the same axis) and a 180° rotation. Each of these subgroups is the symmetry group of a frieze pattern, and sample patterns are shown in Fig. 1. The seven different groups correspond to the 7 infinite series of axial point groups in three dimensions, with n = ∞. They are identified using Hermann–Mauguin notation or IUC notation, orbifold notation, Coxeter notation, and Schönflies notation:
|p1||∞∞||[∞]+||C∞||(hop): Translations only. This group is singly generated, with a generator being a translation by the smallest distance over which the pattern is periodic. Abstract group: Z, the group of integers under addition.|
|p1m1||*∞∞||[∞]||C∞v||(sidle): Translations and reflections across certain vertical lines. The group is the same as the non-trivial group in the one-dimensional case; it is generated by a translation and a reflection in the vertical axis. The elements in this group correspond to isometries (or equivalently, bijective affine transformations) of the set of integers, and so it is isomorphic to a semidirect product of the integers with Z2. Abstract group: Dih∞, the infinite dihedral group.|
|p11m||∞*||[∞+,2]||C∞h||(jump): Translations, the reflection in the horizontal axis and glide reflections. This group is generated by a translation and the reflection in the horizontal axis. Abstract group: Z × Z2|
|p11g||∞×||[∞+,2+]||S∞||(step): Glide-reflections and translations. This group is generated by a glide reflection, with translations being obtained by combining two glide reflections. Abstract group: Z|
|p2||22∞||[2,∞]+||D∞||(spinning hop): Translations and 180° rotations. The group is generated by a translation and a 180° rotation. Abstract group: Dih∞|
|p2mg||2*∞||[2+,∞]||D∞d||(spinning sidle): Reflections across certain vertical lines, glide reflections, translations and rotations. The translations here arise from the glide reflections, so this group is generated by a glide reflection and either a rotation or a vertical reflection. Abstract group: Dih∞|
|p2mm||*22∞||[2,∞]||D∞h||(spinning jump): Translations, glide reflections, reflections in both axes and 180° rotations. This group requires three generators, with one generating set consisting of a translation, the reflection in the horizontal axis and a reflection across a vertical axis. Abstract group: Dih∞ × Z2|
- p1: T (translation only, in the horizontal direction)
- p1m1: TV (translation and vertical line reflection)
- p11m: TH (translation and horizontal line reflection)
- p11g: TG (translation and glide reflection)
- p2: TR (translation and 180° rotation)
- p2mg: TRVG (translation, 180° rotation, vertical line reflection, and glide reflection)
- p2mm: TRHVG (translation, 180° rotation, horizontal line reflection, vertical line reflection, and glide reflection)
The groups can be classified by their type of two-dimensional grid or lattice:
|Rectangular||p1m1, p11m, p11g, p2mm, p2mg|
The lattice being oblique means that the second direction need not be orthogonal to the direction of repeat. The groups' order in this table is their order in the International Tables for Crystallography, which differs from orders given elsewhere.
Web demo and software
There exist software graphic tools that create 2D patterns using frieze groups. Usually, the entire pattern is updated automatically in response to edits of the original strip.
- Kali, a free and open source software application for wallpaper, frieze and other patterns.
- Kali, free downloadable Kali for Windows and Mac Classic.
- Tess, a nagware tessellation program for multiple platforms, supports all wallpaper, frieze, and rosette groups, as well as Heesch tilings.
- FriezingWorkz, a freeware Hypercard stack for the Classic Mac platform that supports all frieze groups.
- Coxeter, H. S. M. (1969). Introduction to Geometry. New York: John Wiley & Sons. pp. 47–49. ISBN 0-471-50458-0.
- Cederberg, Judith N. (2001). A Course in Modern Geometries, 2nd ed. New York: Springer-Verlag. pp. 117–118, 165–171. ISBN 0-387-98972-2.
- Fisher, G.L.; Mellor, B. (2007), "Three-dimensional finite point groups and the symmetry of beaded beads" (PDF), Journal for Mathematics and the Arts
- Radaelli, Paolo G., Fundamentals of Crystallographic Symmetry (PDF)
- Hitzer, E.S.M.; Ichikawa, D. (2008), "Representation of crystallographic subperiodic groups by geometric algebra" (PDF), Electronic Proc. of AGACSE (Leipzig, Germany) (3, 17–19 Aug. 2008)
- Kopsky, V.; Litvin, D.B., eds. (2002), International Tables for Crystallography, Volume E: Subperiodic groups E (5th ed.), Berlin, New York: Springer-Verlag, doi:10.1107/97809553602060000105, ISBN 978-1-4020-0715-6