A robotic surgical instrument comprises an articulation at its shaft's distal end and an instrument interface at its proximal end. The articulation is driveable by pairs of driving elements to articulate an end effector. The instrument interface comprises instrument interface elements configured to drive the driving elements. The instrument interface elements engage drive assembly interface elements of a drive assembly interface of a surgical robot arm when the robotic surgical instrument engages the surgical robot arm. A guide bar is located at an external face of the instrument interface, which, on engaging the instrument interface with the drive assembly interface, is received in the drive assembly interface prior to the instrument interface elements. Once received in the drive assembly interface, the guide bar constrains a relative orientation at which the instrument interface and the drive assembly interface are permitted to engage so as to align their longitudinal attitudes.