Definitions.

1. A point is that which has no part. original: "Punctum est, cuius pars nulla est." Euclid begins with the most basic unit of geometry, defined by its lack of dimension or size.
2. A line is breadthless length. original: "Linea autem sine latitudine longitudo."
3. The extremities of a line are points.
4. A straight line is a line which lies evenly with the points on itself. original: "Recta linea est, quaecunque ex aequo punctis in ea sitis iacet." This suggests a line that does not bend or curve in any direction.
5. A surface is that which has length and width only.
6. The extremities of a surface are lines.
7. A plane surface is a surface which lies evenly with the straight lines on itself. original: "Plana superficies est." This describes a flat surface where any straight line connecting two points stays entirely within that surface.
8. A plane angle is the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line. original: "Planus autem angulus."
9. When the lines containing the angle are straight, the angle is called rectilineal. original: "rectilineus adpellatur angulus."
10. When a straight line set up on a straight line makes—

9. Hero, Definition 17. Boethius, page 374, line 12. 10. Hero, Definition 19. Ammonius, On the Categories, page 58. Simplicius, On Aristotle's On the Heavens, folio 131 verso. Philoponus, On the Physics, book 1, section 4; On the Prior Analytics, book 2, folio 28 verso, page 65. Psellus, page 36. Martianus Capella, Book 6, section 710. Boethius, page 374, line 14.

The numbers for the definitions are omitted in manuscripts P, F, B, and b.
Ammonius, page 66, line 6. Manuscript B uses the phrase has only.
Written in manuscript b. Instead of "in the plane," it reads plane p.
A gap of one letter in manuscripts P and F. 14. Manuscript B uses the elided form but instead of the full word.
Proclus; Manuscript P reads the aforementioned angle.
Proclus.

1. Manuscript F and Psellus use the word nothing.
11. The word but is written as a contraction above the line.
13. There is an erasure before the word toward.
The phrase containing the angle.
15. The phrase the angle is called.