The Fortifications

P R I M O .

The triangle called isosceles is formed with two equal lines, namely the R S, S Q, and its R Q, unequal.
A diagram shows an isosceles triangle labeled with S at the top vertex, R at the bottom left, and Q at the bottom right.

The triangle of three unequal sides, called scalene, is made with three unequal lines, which are T V, V S, S T.
A diagram shows a scalene triangle with vertex V at the top, T at the left, and S at the right.

D I F F I N I T I O N E IX.

The right-angled triangle is formed with the right angle B, by the sides B A, B C, with the diagonal A C, which is half of a perfect square.
A diagram shows a right-angled triangle with the right angle at vertex B, and other vertices labeled A (top) and C (left).

Next is the obtuse-angled triangle, which can be of three unequal sides, and with the right angle E, by the sides E D, D F, F E, which is half of an oblong square.
A diagram shows an obtuse triangle labeled with vertices F (top left), E (bottom left vertex of the obtuse angle), and D (bottom right).

The form G is called a rhombus, formed with two obtuse angles and with two acute angles of equal sides.
A diagram shows a rhombus with the letter G in the center.

D I F F I N I T I O N E X.

The perfect square E is formed with four equal lines, with right angles on each of its sides.
A diagram shows a square with the letter E in the center.

The oblong square F G is formed with two parallel lines, and its ends joined with two other lines perpendicular at right angles.
A diagram shows a rectangle with a mark on the left side and the letter F on the right side.