What is The Checkered Tablecloth

A red and white checked tablecloth graces the table in the Murrys' cozy kitchen. Mr. Murry absentmindedly doodles a scientific equation on the tablecloth one evening in ink. The inked equation fails to come out in the wash, and so it stays on the tablecloth as a reminder to Meg and Mrs. Murry of Mr. Murry's scientific work. The equation on the tablecloth turns out to be an important clue pointing to the involvement of the Echthroi.