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A new relay circuit, applicable to asymmetrical pilot circuits, is described. In the steady state for relays at both ends, the circuit provides identical unit-diameter circular stability areas on the complex plane, centred on (1, 0), and it is suggested that this relay circuit constitutes a preferred characteristic of marginal operation. Certain alternative relay configurations are compared, to ascertain relative susceptibilities to the effects of induced interference voltages and the presence of d.c. components in the relaying quantities derived from the protected system. Homogeneous pilot circuits are assumed for simplicity, and the relay schemes considered include: (i) that referred to above which provides the preferred relay characteristic (ii) that which provides termination of the pilot circuit in characteristic impedance (iii) that which provides compensation for pilot-circuit phase-shift and a self-inverse characteristic.