4. A Line is that which has length merely, and may be straight or curved.
5. A Straight Line, or, as it is sometimes called, a right line, is the shortest, line that can be drawn between two given points. Straight lines are generally designated by letters or figures at their extremities, as A B, Fig. 1.
6. A Curved Line is one which changes its direction at every point, or one of which no portion, howFig. 1 - A S raight Line. ever small, is straight. It is therefore longer than a straight line connecting the same points. Curved lines are designated by letters or figures at their extremities and at intermediate points, as A 15 C or D E F, Fig. 2.
Fig. 2 - Curved Lines.
7. Parallel Lines arc those which have no inclination to each other, being everywhere equidistant. A B and A1 B1 in Fig. '3 are parallel straight Hues, and can never meet though produced to infinity. C D and C D' are parallel curved lines, being arcs of circles which have a common center.
Fig. 3. - Parallel Lines.
8. Horizontal Lines are lines parallel to the horizon, or level. A Horizontal Line in a drawing is indicated by a line drawn from left to right across the paper, as A B in Fig. 4.
Fig. 4. - Names of Lines by Direction.
9. Vertical Lines are lines parallel to a plumb line suspended freely in a still atmosphere. A Vertical Line in a drawing is represented by a line drawn up and down the paper, or at right angles to a horizontal line, as E C in Fig. 4.
10. Inclined or Oblique Lines occupy an intermediate between horizontal and vertical lines, as C D,
Fig. 5. - Perpendicular Lines.
Fig. 4. Two lines which converge toward each other and which, if produced, would meet or intersect, are said to incline to each other.
11. Perpendicular Lines. - Lines are perpendicular to each other when the angles on either side of the point of meeting are equal. Vertical and horizontal lines are always perpendicular to each other, but perpendicular lines are not always vertical and horizontal, but may be at any inclination to the horizon, provided that the angles on either side of the point of intersection are equal. In Fig. 5, C F, D II and E G arc said to be perpendicular to A B. Also in Fig. 6,
Fig. 6. - Perpendicular Lines.
C D and E F are perpendicular to A B. Lines perpendicular to the same line are parallel to each other, as C F and D H, Fig. 5, which are perpendicular to A B.
12. An Angle is the opening between two straight lines which meet one another. An angle is commonly designated by three letters, the letter designating the point in which the straight lines containing the angle meet being between the other two letters, as the angle ECD, Fig. 4.
Fig. 7. - Angles.
13. A Right Angle. - When a straight line meets another straight line so as to make the adjacent angles equal to each other, each angle is a right angle, and the straight lines are said to be perpendicular to each other. (See C B E or C B D, Fig. 7.)
14. An Acute Angle is an angle less than a right angle, as A B D or A B C, Fig. 7.
15. An Obtuse Angle is an angle greater than a right angle, as A B E, Fig. 7. firms and Definitions. 3