In plane geometry, the inclination of a straight line is [0, 180], and two straight lines are parallel or overlap by 0.

Two straight lines intersect (0,90);

In solid geometry, straight lines in different planes of space form angles (0,90); The straight line forms an angle with the plane, which is 0 parallel or in the plane, and the intersection is (0,90); The plane forms an angle with the plane [0,90];

In a vector, the angle is [0, 180]