# ^^Euclide. Definizioni.

Definizioni. | Postulati. | Nozioni comuni.

1. point is that which has no part. >>>
3. ends of a line are points.
4. straight line is a line which lies evenly with the points on itself.
5. surface is that which has length and breadth only.
6. edges of a surface are lines.
7. plane surface is a surface which lies evenly with the straight lines on itself.

8. 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. Rectilinear angle. And when the lines containing the angle are straight, the angle is called rectilinear.
10. Right angle; perpendicular. When a straight line standing 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. obtuse angle is an angle greater than a right angle.
12. acute angle is an angle less than a right angle.

13. boundary is that which is an extremity of anything.
14. figure is that which is contained by any boundary or boundaries.

15. 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 equal one another.
16. And the point is called the center of the circle.
17. diameter of the circle is any straight line drawn through the center and terminated in both directions by the circumference of the circle, and such a straight line also bisects the circle.
18. 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. Rectilinear figures are those which are contained by straight lines,
• trilateral figures being those contained by three,
• quadrilateral those contained by four, and
• multilateral those contained by more than four straight lines.

20. trilateral figures
• equilateral triangle  3 sides equal
• isosceles triangle    2 sides alone equal
• scalene triangle       3 sides unequal.
21. trilateral figures, further
• right-angled triangle  has a right angle
• obtuse-angled triangle has an obtuse angle
• acute-angled triangle has its 3 angles acute.

• square is both equilateral and right-angled
• oblong is right-angled but not equilateral
• rhombus is equilateral but not right-angled
• rhomboid that which has its opposite sides and angles equal to one another but is neither equilateral nor right-angled.
• trapezia quadrilaterals other than these.

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.