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Activity
National Council of Teachers of Mathematics

The Math Forum: What Is a Tessellation?

For Students 3rd - 8th Standards
The Math Forum defines and illustrates geometric tessellation.
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Activity
Wolfram Research

Wolfram Math World: Semiregular Tessellations

For Students 3rd - 8th Standards
This one page Mathworld site has a number of examples of semiregular tessellations. A definition and a small amount of related information is given at the bottom of the page.