The introduction into both the Royal Navy and the Royal Air Force of the air navigational equipment that is at present envisaged emphasizes the need for a reliable and accurate compass. Further, since no compass can be expected to give completely accurate information in all conditions of use, the errors that are likely to arise and their magnitudes must be known. Before an error can be determined, it is first necessary to define what may be regarded as an error, and then to determine its magnitude as accurately as possible. Gone are the days when an expression such as ‘plus or minus two degrees’ would be regarded as satisfactory. The determination of compass accuracy to a tenth of a degree is now an accepted requirement.