# Definition:Euclid's Definitions - Book X

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## Euclid's Definitions: Book $\text{X}$

These definitions appear at the start of Book $\text{X}$ of Euclid's *The Elements*.

- Those magnitudes are said to be
**commensurable**which are measured by the same same measure, and those**incommensurable**which cannot have any common measure. - Straight lines are
**commensurable in square**when the squares on them are measured by the same area, and**incommensurable in square**when the squares on them cannot possibly have any area as a common measure. - With these hypotheses, it is proved that there exist straight lines infinite in multitude which are commensurable and incommensurable respectively, some in length only, and others in square also, with an assigned straight line. Let then the assigned straight line be called
**rational**, and those straight lines which are commensurable with it, whether in length and in square or square only,**rational**, but those which are incommensurable with it**irrational**. - And let the square on the assigned straight line be called
**rational**and those areas which are commensurable with it**rational**, but those which are incommensurable with it**irrational**, and the straight lines which produce them**irrational**, that is, in case the areas are squares, the sides themselves, but in case they are any other rectilineal figures, the straight lines on which are described squares equal to them.

## Sources

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