Shapes can fit together like a puzzle. 

Shapes can fit together like a puzzle. 
Some patterns repeat the same way.
People have used these patterns for a long time. 
Nature makes them too. Bees make hexagons in honeycombs. These shapes fit perfectly together.
Patterns can also be very beautiful. Artists use them to make quilts and art.
Imagine covering a floor with tiles. You want no gaps between them. You also want no overlaps. This way of covering a surface is called a tessellation. 
Some patterns repeat the same way. These are called periodic tilings. You can use just one shape to do this. Only three shapes work perfectly by themselves. These are the equilateral triangle, the square, and the regular hexagon.
Other patterns do not repeat. These are called non-periodic tilings. A famous kind is the Penrose tiling.
People have used these patterns for a long time. The Sumerians used clay tiles for art. 

Imagine covering a floor with beautiful tiles. You want to fit them together perfectly. There should be no gaps between the pieces. There should also be no overlaps where one tile sits on top of another. 

There are different ways these tiles can fit together. A periodic tiling is a pattern that repeats the same way. 
Some patterns are much more mysterious. A non-periodic tiling does not have a repeating pattern. 
People have loved these patterns for thousands of years. Around 4000 BC, the Sumerians used clay tiles for wall decorations. 

You can see tessellations all around you every day. They are not just for math books or art galleries. They can be used in quilts to make pretty designs. 

A tessellation, or tiling, is the process of covering a surface using geometric shapes called tiles. To be a true tessellation, the tiles must fit together perfectly. This means there can be no gaps between the shapes and no overlaps where one tile sits on top of another. 

To understand how these patterns work, we must look at the relationship between tiles. An edge is the line where two bordering tiles meet. A vertex is the specific point where three or more tiles intersect. 
Tessellations are categorized by how much they repeat. A periodic tiling follows a pattern that repeats regularly. These patterns can be organized into seventeen distinct mathematical categories called wallpaper groups. 
We can further classify tilings by the types of polygons they use. A regular tessellation is highly symmetric and uses only one type of regular polygon. Only three shapes can form a regular tessellation: the equilateral triangle, the square, and the regular hexagon. A semi-regular, or Archimedean, tessellation uses more than one type of regular polygon. However, it must be isogonal, meaning every vertex looks exactly the same. There are eight known types of semi-regular tessellations. For example, a tiling might have a vertex configuration of 4.8.8, meaning one square and two octagons meet at every corner.
History shows that humans have used these geometric principles for millennia. Around 4000 BC, the Sumerians created wall decorations using clay tiles. 

The significance of tessellations extends from art to the natural world. In nature, bees create hexagonal cells in honeycombs to organize their hives. 

Mathematics continues to find new complexities in these patterns. The Swiss geometer Ludwig Schläfli pioneered the study of higher-dimensional versions of these shapes, called polytopes. He created the Schläfli symbol notation to describe them compactly. For instance, a tiling of regular hexagons can be written as {6,3} to show it has six-sided polygons meeting in groups of three. While many rules exist, such as the Conway criterion for periodic tiling, no general rule has been found to determine if any given shape can tile a plane. This leaves many interesting problems for mathematicians to solve.
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