Book 1
DEFINITIONS.
1
1
A point is that which has no part.
2
2
A line is breadthless length.
3
3
The extremities of a line are points.
4
4
A straight line is a line which lies evenly with the points on itself.
5
5
A surface is that which has length and breadth only.
6
6
The extremities of a surface are lines.
7
7
A plane surface is a surface which lies evenly with the straight lines on itself.
8
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
9
And when the lines containing the angle are straight, the angle is called rectilineal.
10
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
11
An obtuse angle is an angle greater than a right angle.
12
12
An acute angle is an angle less than a right angle.
13
13
A boundary is that which is an extremity of anything.
14
14
A figure is that which is contained by any boundary or boundaries.
15
