### Use and Choice of CTYPEi keywords

When reading a FrameSetFrameSet from a FitsChanFitsChan the CTYPEi values in the FitsChan are used to set the Symbol attributes of the corresponding WCS FrameFrame. The Label attributes of the WCS Frame are set from the CNAMEi keywords, if present in the header. Otherwise they are set from the CTYPEi comments strings in the header, so long as each axis has a unique non-blank comment. Otherwise, the Label attributes are set to the CTYPEi values. The above procedure is over-ridden if the axis types conform to the conventions described in paper II or III, as described below.

When writing a FrameSet to a FitsChan, each CTYPEi value is set to the value of the Symbol attribute of the corresponding axis in the Frame being written. If a value has been set explicitly for the axis Label attribute, it is used as the axis comment (except that any existing comments in the FitsChan take precedence if the keyword value has not changed). The above procedure is over-ridden if the Frame is a SkyFrameSkyFrame or a SpecFrameSpecFrame, in which case the CTYPEi value is derived from the SystemSystem attribute of the Frame and the nature of the pixel to WCS MappingMapping according to the conventions of papers II and III, as described below.