Template

A template is a piece of flat metal, wood, plastic, paper or cardboard used as a pattern for outlining a shape.
 
How to Make a Square Decimeter Template
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It took me about ten minutes to rip a one decimeter = (10cm), on a table saw, (drg.#1),   and cut it to length (drg #2) using a radial arm saw.               

The radial arm saw is great because I clamped a back stop 10cm from the blade and cut four unfinished pieces of hardboard. Drg#3 This is easy to make and I had about 30 in my classroom.

I also went outside and sprayed painted the unfinished pieces with chalkboard paint.
Drg #4