Relay "trees" and symmetric circuits
S. H. Washburn · Transactions of the American Institute of Electrical Engineers · 1949
THE RELAY “tree,” so-called because of its configuration, is a special case of a multiterminal network of relay contacts in which a single input terminal may be connected to one of a number of output terminals. The path from each output terminal to the input end of the network passes through contacts on all the relays upon which the network is constructed; and no paths can be connected together. The particular path closed through depends upon the combination of operated relays.