The unwritten fifth word “Fir” was key to finding the stolen jewellery because it indicated the jewellery’s hiding place—a jug of earth with a young fir tree. While Nolan wrote a four-word code for the days Monday through Thursday, the fifth day, Friday, was symbolized by “Fir.” Understanding this missing piece was essential for solving the mystery. Encyclopedia’s insight into this subtle clue demonstrated his sharp reasoning skills and helped bring the case to a successful conclusion.