Projections of Plane Figures
A plane surface enclosed or bounded by straight lines or a curved line is called a plane figure. The least number of straight lines which can enclose a space is three. When three straight lines intersect, the plane surface enclosed is a triangle. With four straight lines, the figure obtained is a quadrilateral. The most common plane figures are set squares (30°–60° and 45°). Other common plane figures include square, rectangle, pentagon, hexagon, octagon, circle, ellipse, etc.