Inclination of a Line

5.2.1 Inclination of a Line

(This sub-section is printed as "5.1.2" a second time in the source scan — an evident numbering slip, since it falls after 5.2's intercept discussion and before 5.2.2 Slope; it is presented here at its correct sequential position, 5.2.1.)

The inclination of a line is the smallest angle it makes with the positive direction of the X-axis, measured in the anticlockwise sense. It is denoted $\theta$, and by definition $0^\circ \le \theta < 180^\circ$.

Remark. Two lines are parallel if and only if they have the same inclination — this is intuitively clear, since "same tilt relative to the X-axis" is exactly what parallel means. …