Instrument interface for robotic surgical instrument

A robotic surgical instrument, comprising: a shaft; an end effector element; an articulation at a distal end of the shaft for articulating the end effector element, the articulation comprising: a first and second joint permitting the end effector element to adopt a range of configurations relative to a longitudinal axis of the shaft, the first joint being driveable by a first pair of driving elements having a first positional accuracy requirement and the second joint being driveable by a second pair of driving elements having a second positional accuracy requirement lower than the positional accuracy requirement of the first pair of driving elements; and an instrument interface at a proximal end of the shaft, comprising: a chassis formed from the attachment of a first chassis portion to a second chassis portion, the first chassis portion comprising a mounting surface to which the shaft is mounted; wherein the first pair of driving elements are secured relative to the first chassis portion.