## The SpecFrame Model

As for a SkyFrameSkyFrame, a SpecFrameSpecFrame is a FrameFrame (§7) and also a MappingMapping (§5), so it inherits all the properties and behaviour of these two ancestral classes. When used as a Mapping, a SpecFrame implements a unit transformation, exactly like a basic Frame (§7.3) or a UnitMapUnitMap, so this aspect of its behaviour is not of great importance.

When used as a Frame, however, a SpecFrame represents a wide range of different 1-dimensional coordinate system which can be used to describe positions within a spectrum. The options available largely mirror those described in the FITS-WCS paper III Representations of spectral coordinates in FITS (Greisen, Valdes, Calabretta & Allen).