Table of contents |

2 Basic descriptions 3 Examples 4 Orbifold notation 5 Related Topics 6 External links |

There are seventeen different types of wallpaper patterns. As opposed to the frieze patterns, these patterns cover the entire plane and can be extended infinitely in any direction on the plane. Discrete frieze patterns extend infinitely in only one direction, and not across the entire plane.

An isometry or *rigid motion* in the plane is a map *m* : **R**^{2} → **R**^{2} such that *m* is distance-preserving.
i.e. *d*(*u*,*v*) = *d*(*m*(*u*),*m*(*v*)) where *d*((*u*_{1},*u*_{2}),(*v*_{1},*v*_{2})) = sqrt((*u*_{1}−*v*_{1})^{2} + (*u*_{2}−*v*_{2})^{2}).

All isometries in the plane are bijections and are obtained from combinations of:

translations by *a* = (*a*_{1},*a*_{2}): *t*_{a}(*u*) = *u*+*a* where + is vector addition

rotations about the origin: *ρ*_{θ}(*u*_{1},*u*_{2}) = (*u*_{1}cos*θ*−*u*_{2}sin*θ*,*u*_{1}sin*θ*+*u*_{2}cos*θ*)

reflections about the *x* axis: *r*(*u*_{1},*u*_{2}) = (*u*_{1},−*u*_{2})

In fact each isometry is one of the following four types

- translation (as above)
- rotation by
*θ*about*a*:*ρ*_{θ}(*u*−*a*)+*a* - reflection about any line
*l*in**R**^{2}:*r*_{a,p}where*a*is a vector in the direction of the line,*p*is a point on the line - glide reflection about line
*l*:*g*_{a,p}(*u*) =*t*_{a}(*r*_{a,p}(*u*))

To decide which of the 17 classes any particular plane periodic pattern belongs to, determine the rotational symmetry and whether or not the pattern has reflection symmetry or nontrivial glide-reflection symmetry. If necessary, the last step is to determine the locations of the centers of rotation.

The seventeen wallpaper patterns can be described as follows:

- No rotation, at least one reflection, and a glide-reflection that is not on the reflection axis (cm)
- No rotation, at least one reflection, no glide-reflections that can be located outside of the reflection axis (pm)
- Glide-reflections only (pg)
- No rotations, no reflections, no glide-reflection (only translations) (p1)
- Rotation of 180 degrees located on the reflection axis, reflections in two directions (pmm)
- Rotation of 180 degrees not on a reflection axis, reflections in two directions (cmm)
- Rotation of 180 degrees, reflections only in one direction (pmg)
- Rotation of 180 degrees, a glide-reflection (pgg)
- Rotation of 180 degrees only (p2)
- Rotation of 90 degrees, reflections in four directions (p4m)
- Rotation of 90 degrees, reflections in less than four directions (p4g)
- Rotation of 90 degrees only (p4)
- Rotation of 120 degrees, reflections with a threefold center located on the axis (p3m1)
- Rotation of 120 degrees, reflections with a threefold center not located on the axis (p31m)
- Rotation of 120 degrees only (p3)
- Rotation of 60 degrees, at least one reflection (p6m)
- Rotation of 60 degrees only (p6)

A generating region of a periodic pattern is the smallest portion of the lattice unit whose images under the full symmetry group of the pattern cover the plane.

http://www.xahlee.org/Wallpaper_dir/c0_WallPaper.html The Discontinuous Groups of Rotation and Translation in the Plane