# Definition:Euclid's Definitions - Book I/20 - Triangles by Sides

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## Definition

In the words of Euclid:

*Of trilateral figures, an***equilateral triangle**is that which has its three sides equal, an**isosceles triangle**that which has two of its sides alone equal, and a**scalene triangle**that which has its three sides unequal.

(*The Elements*: Book $\text{I}$: Definition $20$)

## Sources

- 1926: Sir Thomas L. Heath:
*Euclid: The Thirteen Books of The Elements: Volume 1*(2nd ed.) ... (previous) ... (next): Book $\text{I}$. Definitions