In a previous paper the writer considered the problems of establishing latitude at sea in the sixteenth century and in part based his conclusions on the results of experiments made with modern replicas of the quadrant, the mariner's astrolabe and in particular the cross-staff, these being the instruments in use at the time. Reference was made to the backstaff or Davis quadrant which, though first described by John Davis in its simplest form in 1595 was not in effective use in its final version until well into the seventeenth century. It remained, together with the cross-staff, the principal altitude-measuring instrument in use at sea until the introduction of instruments of reflection in the eighteenth century and indeed continued in use long after that time, no doubt because it was cheaper than the new instruments, but perhaps also in part because of the ingrained conservatism of seafarers. The name ‘backstaff’ can be used generically to refer to all those instruments with which the observer turns his back on the Sun (e.g. the Gunter's bow), but they all post-dated and derived in concept from the Davis quadrant, which remained the most common in England and elsewhere. (The French knew it as le quartier anglais.) It is in this sense that the term is used here.