ELEMENTS OF GEOMETRY, or Definitions.

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A Point is that which cannot be divided, whose part is nothing.

A Line is a length without breadth or thickness, whose extremities are two points.

The shortest line drawn between two points is a straight line; the contrary are crooked lines.

Three geometric diagrams on one line: a small dot labeled "A point"; a straight horizontal line labeled "A right line"; and two intersecting curved lines forming a lens shape, labeled "Crooked lines".

A Superficies (surface) is that which has length and breadth only, being bounded or determined with lines.

A Plain Superficies is that which lies equally and evenly between its lines or bounds.

A rectangular plane with horizontal shading/hatching to represent a surface, labeled above as "A superficies."

A Plain Angle is the inclination of two lines lying in one plain superficies, concurring or meeting in a point.

If those lines that contain the angle be straight, it is called a right-lined angle, and those two lines are its containing sides; but if a third straight line be drawn across the former from one to the other, that shall be called the subtending side.

A right-angled triangle with vertices labeled A (top), B (bottom left corner), and C (bottom right). Text is printed within the triangle area describing the parts of the figure.

A
ABC the contained angle
AB BC the containing sides
AC the side subtending
B C