Orthographic views are two-dimensional views of three-dimensional objects.  How the views are laid out on a drawing depends on which projection method is being used - First angle projection or Third angle projection. Only one projection method is allowed to be used on any given drawing. A symbol, sometimes referred to as the "plug symbol," is placed on the drawing to identify which of the two methods is being used.

We must always identify which method is used when we are required to interpret a drawing. Not doing so can result in incorrect manufacture, inspection, and ultimately, scrapped parts.