When I look at the painting Nighthawks by Edward Hopper, the first thing that I notice is the loneliness of it all. The couple seen in the painting sit together, but the way the lines divide them from the others gives the appearance of isolation. In The Decameron's fourth day, first story, I feel this same isolation in the story of Ghismonda and Guiscardo. Ghismonda's father, Tancredi, refuses to allow her to marry, so she must hide her love for Guiscardo, along with their subsequent affair. They are together, but their togetherness must be hidden, and in this way they are isolated. Tancredi eventually finds out about their affair, however. He kills Guiscardo in retaliation and gives Guiscardo's heart to Ghismonda. She cannot bear to be alone, so she drinks his heart-blood mixed with poison to be with him in death. 

The couple in this painting seem to also be together, they are sitting quite close and drinking together. however, they are also alone. The are separated from each of the other two, and the rest of the painting is just empty space. However, the way the bar sits perpendicular to the wall draws the focus to these two. Whether they found each other in the diner that night or they're a married couple, their presence late at night in a near empty diner says that although they may be alone, they are alone together and seem to prefer it that way.