 Recognising that the prime factors of this number are 2, 7 and 41, he tried again with a period of 41 and "got a rectangle of dots and crosses that was replete with repetitions".It was clear, however, that the first impulse of the key was more complicated than that produced by a single wheel of 41 key impulses.Tutte called this component of the key formula_11 ("chi"1).