The angle a line makes with the positive x-axis, measured anticlockwise, is its:

1. A line is fixed in direction by the angle it makes with a reference. 2. That reference is the positive direction of the x-axis. 3. The angle measured anticlockwise from it is called the inclination. 4. It runs from zero up to one hundred and eighty degrees. _Source: syllabus Mathematics, Ch 9 'Straight Lines'_