As  a House Painter I have learned that making straight lines is an art.  First nothing is straight,  the Framing and 2 by fours are not straight, the drywall is not hung straight, and the taping of the corners are not straight.  So as the painter it is my job to create a straight line.  Using a straight edge or a level will not work because then you are straightening out a corner that was not straight to begin with!!   Creating a line that looks straight is a work of art and like a painting may take a couple coats from different directions to make it possible.  This is the reality of painting and it is a challenge that I relish in:)