A point is that which has no parts.
A line is length without breadth.
The extremities of a line are points.
A ſtraight or right line is that which lies evenly between its extremities.
A ſurface is that which has length and breadth only.
The extremities of a ſurface are lines.
A plane ſurface is that which lies evenly between its extremities.
A plane angle is the inclination of two lines to one another, in a plane, which meet together, but are not in the ſame direction.
A plane rectilinear angle is the inclination of two ſtraight lines to one another, which meet together, but are not in the ſame ſtraight line.
When one ſtraight line ſtanding on another ſtraight line makes the adjacent angles equal, each of theſe angles is called a right angle, and each of theſe lines is ſaid to be perpendicular to the other.
An obtuſe angle is an angle greater than a right angle.
An acute angle is leſs than a right angle.
A term or boundary is the extremity of any thing.
A figure is a ſurface encloſed on all ſides by a line or lines.
A circle is a plane figure, bounded by one continued line, called its circumference or periphery; and having a certain point within it, from which all ſtraight lines drawn to its circumference are equal.
This point (from which the equal lines are drawn) is called the centre of the circle.
A diameter of a circle is a ſtraight line drawn through the centre, terminated both ways in the circumference.
A ſemicircle is the figure contained by the diameter, and the part of the circle cut off by the diameter.
A ſegment of a circle is a figure contained by a ſtraight line, and the part of the circumference which it cuts off.
A figure contained by ſtraight lines only, is called a rectilinear figure.
A triangle is a rectilinear figure included by three ſides.
A quadrilateral figure is one which is bounded by four ſides. The ſtraight lines and connecting the vertices of the oppoſite angles of a quadrilateral figure, are called its diagonal.
A polygon is a rectilinear figure bounded by more than four ſides.
A triangle whoſe three ſides are equal, is ſaid to be equilateral.
A triangle which has only two ſides equal is called an iſoſceles triangle.
A ſcalene triangle is one which has no two ſides equal.
A right angled triangle is that which has a right angle.
An obtuſe angled triangle is that which has an obtuſe angle.
An acute angled triangle is that which has three acute angles.
Of four-ſided figures, a ſquare is that which has all its ſides equal, and all its angles right angles.
A rhombus is that which has all its ſides equal, but its angles are not right angles.
An oblong is that which has all its angles right angles, but has not all its ſides equal.
A rhomboid is that which has its oppoſite ſides equal to one another, but all its ſides are not equal, nor its angles right angles.
All other quadrilateral figures are called trapeziums.
Parallel ſtraight lines are ſuch as are in the ſame plane, and which being produced continually in both directions, would never meet.