use angle, as A, Figure 62.

When all the sides of a triangle are equal in length and the angles are
all equal, as in Figure 63, it is termed an equilateral triangle, and
either of its sides may be called the base. When two only of the sides
and two only of the angles are equal, as in Figure 64, it is termed an
isosceles triangle, and the side that is unequal, as A in the figure, is
termed the base.

[Illustration: Fig. 65.]

[Illustration: Fig. 66.]

When all the sides and angles are unequal, as in Figure 65, it is termed
a scalene triangle, and either of its sides may be called the base.

The angle opposite the base of a triangle is called the vertex.

[Illustration: Fig. 67.]

[Illustration: Fig. 68.]

A figure that is bounded by four straight lines is termed a quadrangle,
quadrilateral or tetragon. When opposite sides of the figure are
parallel to each other it is termed a parallelogram, no matter what the
angle of the adjoining lines in the figure may be. When all the angles
are right angles, as in Figure 66, the figure is called a rectangle. If
the sides of a rectangle are of equal length, as in Figure 67, the
figure is called a square. If two of the parallel sides of a rectangle
are longer than the other two sides, as in Figure 66, it is called an
oblong. If the length of the sides of a parallelogram are all equal and
the angles are not right angles, as in Figure 68, it is called a rhomb,
rhombus or diamond. If two of the parallel sides of a parallelogram are
longer than the other two, and the angles are not right angles, as in
Figure 69, it is called a rhomboid. If two of the parallel sides of a
quadrilateral are of unequal lengths and the angles of the other two
sides are not equal, as in Figure 70, it is termed a trapezoid.

[Illustration: Fig. 69.]

[Illustration: Fig. 70.]

[Illustration: Fig. 71.]

If none of the sides of a quadrangle are parallel, as in Figure 71, it
is termed a trapezium.

THE CONSTRUCTION OF POLYGONS.

[Illustration: Fig. 71_a_.]

[Illustration: Fig. 72.]

The term polygon is applied to figures having flat sides equidistant
from a common centre. From this centre a circle may be struck that will
touch all the corners of the sides of the polygon, or the point of each
side that is central in the length of the side. In drawing a polygon,
one of these circles is used upon which to divide the figure into the
requisite number of divisions for the sides. When the dimension o