A robotic surgical instrument comprising: a shaft; an articulation at a distal end of the shaft for articulating an end effector, the articulation driveable by a pair of driving elements; and an instrument interface at a proximal end of the shaft, the instrument interface comprising an instrument interface element for driving the pair of driving elements, the instrument interface element displaceable over a first displacement range, the instrument interface element comprising a body receivable in a drive assembly interface element of a robot arm when the robotic surgical instrument engages with the surgical robot arm, the drive assembly interface element displaceable over a second displacement range, the length of the body in the displaceable direction being greater than one of the maximum travel of the body over the first displacement range and the maximum travel of the drive assembly interface element over the second displacement range.