The OpenGL functions glBegin and glEnd are used in pairs. glBegin marks the start of the section that defines the geometry to be displayed while glEnd marks the end of the section. The type of basic geometry that we want to draw is established by a OpenGL constant passed as a parameter to glBegin (e.g., GL_LINE_LOOP). We study the basic geometries of OpenGL with the tutorial Geometry. The OpenGL function calls written between glBegin and glEnd are at minimum calls that define the vertices of the geometry to be drawn.