Read four lines that are parallel with _e f_. Why are the lines _e f_ and _g h_ said to be parallel to each other? When are lines said to be parallel to each other? Read four lines that are parallel to 5 6. Four that are parallel to _o p_. Is any line parallel to _u v_? Can a single line be properly called perpendicular? Parallel? If two lines are perpendicular to each other, what angle do they form? If parallel, what angle? If oblique? [Illustration: Diagram 13.] LESSON THIRTEENTH. RELATIONS OF ANGLES. INTERIOR ANGLES. Is the angle _a m n_ between the parallels, or outside of them? (DIAGRAM 13.) It is called an _interior angle_. Read three other interior angles between the same parallels. Why is _b m n_ an interior angle? _An interior angle is one that lies between parallel lines._ Read the interior angles between the parallel lines _g h_ and _k l_. Why is _o p l_ an interior angle? What is an interior angle? EXTERIOR ANGLES. Is the angle _a m e_ between the parallels, or outside of them? It is called an _exterior angle_. Read three other exterior angles formed by the lines _a b_, _c d_, and _e f_. Why is the angle _c n f_ an exterior angle? _An exterior angle is one that lies outside of the parallels._ LESSON FOURTEENTH. REVIEW. Read all the interior angles formed by the lines _a b_, _c d_, and _e f_. Why is _m n d_ an interior angle? What is an interior angle? Read all the exterior angles formed by the same lines. Why is _d n f_ an exterior angle? What is an exterior angle? Read all interior angles formed by the lines _g h_, _k l_, and _i j_. All the remaining interior angles in the diagram. All the exterior angles. [Illustration: Diagram 14.] LESSON FIFTEENTH. RELATIONS OF ANGLES. OPPOSITE ANGLES. Are the angles _e m b_, _b m n_, on the same side of the intersecting line _e f_? Are they adjacent? Are _e m b_, _m n d_, on the same side of the intersecting line _e f_? Are they adjacent? Then they are called opposite angles. _Opposite angles lie on the same side of the intersecting line, but are not adjacent._ Are the angles _e m b_, _f n d_, on the same side of the intersecting line? Are they adjacent? Then are they opposite? Are they interior or exterior angles? Then they are “_opposite exterior angles_.” Why are they exterior? Why are they opposite? Are the angles _b m n_, _m n d_, opposite angles? Are they interior or exterior angles? Then they are “_opposite interior angles_.” Why are they opposite? Why interior? Read the opposite exterior angles on the left of the line _e f_.