Although they have not been formally endorsed by the IAUFWG, the AIPS conventions (Greisen 1983) have been widely used, in particular for distribution of IRAS infrared and Einstein X-ray imagery. A detailed discussion and analysis appears in AIPS memo 27 (Greisen 1983). Hanisch and Wells (1988; hereafter HW) proposed adopting these conventions as standard within FITS. They also discussed a number of other conventions recommended at a meeting held in Charlottesville, Virginia, in January 1988, sponsored by NASA Code EZ. The GC proposal marks further movement in the directions begun at Charlottesville. The discussion in the remainder of section 4 of the User's Guide is based primarily upon those three documents, especially GC. However, it does not necessarily reflect the views of all the cited authors on all points.