Are the angles _a e c_, _b e d_ formed by the same straight lines? Are they adjacent angles? They are called “vertical angles.” Vertical angles are angles formed by the same straight lines, but not adjacent to each other. Read the other pair of vertical angles formed by the lines _a b_, _c d_. Read all the vertical angles formed by the lines _f g_, _i h_. By _l m_, _n p_. Why are the angles _l o n_, _n o m_ adjacent angles? Why are the angles _l o n_, _p o m_ vertical angles? [Illustration: Diagram 8.] LESSON EIGHTH. REVIEW. Read the pairs of adjacent angles above the line _a b_. (DIAGRAM 8.) Why are they adjacent? What are adjacent angles? Read the adjacent angles below the line _a b_. On the right of the line _c d_. On the left. How many pairs of adjacent angles are formed by the intersection of two lines. Read the pairs of adjacent angles formed by the lines _f g_ and _i h_. Read all the adjacent angles formed by the lines _l m_, _n p_. Read all the pairs of vertical angles formed by the lines _a b_, _c d_. Why are _c e b_ and _a e d_ called vertical angles? What are vertical angles? Read all the pairs of vertical angles formed by the lines _h i_, _f g_. How many pairs of vertical angles are formed by the intersection of two lines? Read all the pairs of vertical angles formed by the lines _l m_, _n p_. LESSON NINTH. KINDS OF ANGLES. RIGHT ANGLES. What do we call the angles _a o c_, _c o b_? (DIAGRAM 9.) Are they equal to each other? Then they are called _right angles_. _A right angle is one of two adjacent angles that are equal to each other._ Are the adjacent angles _c o b_, _b o d_ equal to each other? Then what are they called? Read the right angles below the line _a b_. On the left of _c d_. Read three right angles whose vertices are at _p_. [Illustration: Diagram 9.] ACUTE ANGLES. Is the angle _m p q_ greater or less than the right angle _m p r_? Then it is called an _acute angle_. _An acute angle is one which is less than a right angle._ Read four acute angles whose vertices are at _p_. Acute means sharp. Why is _r p s_ an acute angle? What is an acute angle? OBTUSE ANGLES. Is the angle _m p s_ greater or less than the right angle _m p r_? Then it is called an _obtuse angle_. _An obtuse angle is one which is greater than a right angle._ What other obtuse angle has its vertex at _p_? Obtuse means blunt. Read three obtuse angles whose vertices are at _x_. Acute and obtuse angles are also called oblique angles. LESSON TENTH. REVIEW.