Read all the right angles formed by the lines _a b_ and _c d_. (DIAGRAM 10.) Why are the adjacent angles _c e b_, _b e d_, right angles? What is a right angle? Read four right angles whose vertices are at _n_. Which is the greater, the right angle _p q r_, or the right angle _t s u_? Can one right angle be greater than another? Read six acute angles whose vertices are at _n_. Why is _m n g_ an acute angle? What is an acute angle? Which is greater, the acute angle _m n g_, or the acute angle _l n m_? May one acute angle be greater than another? What three acute angles are equal to one right angle? [Illustration: Diagram 10.] Which of the two acute angles _v f w_, _y x z_ is the greater? Read four obtuse angles whose vertices are at _n_. Why is _f n m_ an obtuse angle? What is an obtuse angle? What does obtuse mean? Acute? By what other name are both called? Which is greater, the large acute angle 1 4 2, or the small obtuse angle 1 4 3? How much greater than the right angle is the obtuse angle _f n l_? How much less than a right angle is _f n i_? [Illustration: Diagram 11.] LESSON ELEVENTH. RELATIONS OF LINES. PERPENDICULAR LINES. What kind of angles do the lines _a b_ and _c d_ make with each other? (DIAGRAM 11.) Then they are perpendicular to each other. What line is perpendicular to _x y_? Why is it perpendicular to it? What line is perpendicular to _z_ 1? When is a line said to be perpendicular to another? Can a line standing alone be properly called a perpendicular line? What two lines are perpendicular to the lines _r s_? Is the line _g h_ perpendicular to the line _i j_? Why? What other line is perpendicular to the line _i j_? Read three lines that are perpendicular to the line _a b_. PARALLEL LINES. Do the lines _k l_, _m n_, differ in direction? Then do they form any angle with each other? They are said to be _parallel_ to each other. Read four other lines that are parallel with _k l_. What line is parallel with 2 10? Why? _Lines are parallel with each other when they do not differ in direction._ OBLIQUE LINES. What kind of angles do the lines _u t_ and 8 9 form with each other? Then they are said to be oblique to each other. _Lines are oblique to each other when they form oblique angles._ See Note C, Appendix. [Illustration: Diagram 12.] LESSON TWELFTH. REVIEW. Read five lines that are perpendicular to the line _a b_. (DIAGRAM 12.) Five that are perpendicular to _c d_. Two that are perpendicular to _u v_, and meet it. Three that do not meet it. Why are _o p_ and _m n_ perpendicular to each other? When are lines said to be perpendicular to each other?