Formulae are given for the transmission of rotating sector disks for which the edges of the sectors are not radial with respect to the centre of rotation of the disk. The cases considered include single-aperture disks for beams of various cross-sectional forms and multi-aperture disks for which the edges of the sectors are centred on the same and different points. The formulae indicate how errors due to lack of correct centring can be minimized by suitable construction of the disk.It is concluded that the errors likely to arise in practice will be negligible for all except single-aperture disks, and disks the apertures of which are not symmetrically placed about a common centre, or for the most precise work. Where significant the errors can be estimated with sufficient accuracy from the approximate formulae given. These conclusions have been confirmed by experimental test.