Straight lines, semi-straight lines, and segments
AngleAn angle is formed by two semi-straight lines with same vertex .
Straight angleA right angle is an angle which has the same shape as angles in squares or rectangles.
Parallel, perpendicular linesTwo straight lines are parallel if they never intersect.
Two straight lines are perpendicular if they intersect with a right angle.
See also : geometry on fmaths.com