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Branched ducts can produce a range of resonances and antiresonances, which may be varied by changing the termination condition. One example of the use of such branching is the forked didjeridu or didjeriduo, an unusual instrument occasionally made when a forked section of a tree is suitably eaten by termites. A single player may select the mouthpiece, then produce changes in pitch and timbre, either by adjusting lip tension to select different bore resonances, or by using the heel of his hand to close the other mouthpiece. It is even possible for two players to play the same instrument simultaneously. Here we present detailed measurements of the acoustic input impedance of a forked didjeridu and employ numerical modelling to explain the major features. The modelling gives insights into the behaviour of branched ducts in general.