The codebreakers tried to work out the details of the machine from this information, but at first they failed. They were then joined by a young codebreaker called Bill Tutte who was given the job. After a lot of work he succeeded, and produced a logical description of the unseen machine. This work has been described as "the greatest intellectual achievement of world war II". Tutte worked out that the machine made each keystream character by combining the effects of two sets of five wheels. He used Greek letters to name the wheels. He called one set of five the χ ("chi") wheels, and the other set of five the ψ ("psi") wheels. He worked out that the χ wheels moved on one position for each new character coded. The ψ wheels, however, did not move regularly. They only moved on some of the time. Whether or not the ψ wheels moved on was controlled by two wheels that he called two the μ ("mu") or "motor wheels".