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BOOK I.

DEFINITIONS.

1

A point is that which has no part.

2

A line is breadthless length.

3

The extremities of a line are points.

4

A straight line is a line which lies evenly with the points on itself.

5

A surface is that which has length and breadth 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.

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.

9

And when the lines containing the angle are straight, the angle is called rectilineal.

10

When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands.

11

An obtuse angle is an angle greater than a right angle.

12

An acute angle is an angle less than a right angle.

13

A boundary is that which is an extremity of anything.

14

A figure is that which is contained by any boundary or boundaries.

15

A circle is a plane figure contained by one line such that all the straight lines falling upon it from one point among those lying within the figure are equal to one another;

16

Euclid. Euclid's Elements. Sir Thomas Little Heath. New York. Dover. 1956.

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