Euclid's Elements
Book I Translation Project
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 of boundaries. 
15. 
A circle is a plane figure contained by one line such that all the straight lines falling from one point among those lying within the figure are equal to one another; 
16. 
And the point is called the center of the circle. 
17. 
A diameter of the circle is any straight line drown through the center and terminated in both directions by the circumference of the circle, and such a straight line also bisects the circle. 
18. 
A semicircle is the figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same as that of the circle. 
19. 
Rectilineal figures are those which are contained by straight lines: trilateral figures (triangles) being those contained by three; quadrilateral, those contained by four; and multilateral, those contained by more than four straight lines. 
20. 
Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle, that which has two of its sides equal, and a scalene triangle, that which has its three sides unequal. 
21. 
Further, of trilateral figures, a rightangled triangle is that which has a right angle, and obtuseangled triangle is that which has an obtuse angle, and an acuteangled triangle is that which has its three angles acute. 
22. 
Of quadrilateral figures, a square is that which is both equilateral and rightangled; an oblong is that which is rightangled but not equilateral; a rhombus is that which is equilateral but not rightangled; and a rhomboid is that which has its opposite sides and angles equal to one another but is neither equilateral nor rightangled. And let quadrilateral other than these be called trapezia. 
23. 
Parallel straight lines are straight lines which, being in the same plane and being produced indefinitely in both directions, do not meet one another in either direction. 

Let it have been assumed that 
POSTULATES

1. 
A finite straight line may be drawn from any point to any other point; 
2. 
A finite straight line may be continuously extended upon a straight line; 
3. 
With any center and diameter a circle may be described; 
4. 
All right angles are equal to one another; 
5. 
If a straight line placed across two straight lines should make the interior angles on the same side less than two right angles, then the two straight lines, being extended without end, will meet on the side of the crossing line on which the angles are less than the two right angles; 
COMMON NOTIONS

1. 
Things equal to the same thing are also equal to one another; 
2. 
If to equals equals are added, the wholes are equal; 
3. 
If from equals equals are subtracted, the remainders are equal; 
4. 
Things which coincide with one another are equal to one another; 
5. 
The whole is greater than a part [of it]. 
Project Home Page
Book I Propositions
