Entanglement refers to the correlation of a subsystem with its complement. It is an essential resource in quantum information and many-body physics. I will discuss the entanglement of free Fermions on graphs. I will focus on those of the Hamming and Johnson schemes. I shall make use of the parallel with time and band limiting problems in signal processing. I will indicate the role of the Terwilliger algebra in the identification of an operator of Heun type that commutes with the truncated correlation matrix and allows access to the entanglement entropy.