Task-analysis techniques (Gagné, 1962, 1967) were used to generate hypotheses about the order of difficulty of tasks involving a sonometer, a physical system in which the pitch of a note is reciprocally dependent on the length and tension of a vibrating string. Predictions relating task performance to level of mental maturity were formulated on the basis of results reported by Inhelder & Piaget (1958) for similar systems. Administration of the tasks to 42 children aged 6–12 yr. provided general confirmation of the predictions for task difficulty and age-placement. An analysis of developmental trends across task blocks is reported. It is concluded that behaviour in this situation conforms to the behaviour of concrete operational children in related situations.