In an orthographic projection, the invisible and interior details of an object are usually represented by dotted lines. When these dotted lines are numerous, the drawing is difficult to interpret and it becomes almost impossible to read it clearly. To overcome this difficulty, usually it is imagined that the object is being cut through completely or partially by a plane called cutting or section plane and the portion of the object between the section plane and the observer is assumed to be removed to show the interior details of the object clearly.
Section or Cut surface: the surface obtained by cutting an object by the section plane is called section or cut surface.
Section planes: the section planes are assumed to be thin, transparent, perpendicular to one of the reference planes and either parallel or inclined or perpendicular to the other.
True shape of section: The true form of the cut surface is called true shape of section. It is obtained by viewing the object normal to the cut surface and projecting it on a plane parallel to the section plane.
Section lines: the cut surface of the object is shown by a series of thin, inclined and parallel lines called section lines. This process is called hatching.