Localized curvature of a surface that is otherwise flat is sometimes called a curvature defect. If we put a cube on a table and cover it with a table cloth and wrap it with three rubber bands, the top four corners are called −90° defects and the bottom four corners are called +90° defects.
Here is my question: How does one visualize and draw the two kinds of defects in the plane surrounding lattice squares so that we can use the drawing to explain the two terminologies?