The values "left", "top", "right" and "bottom" (or any abbreviation) can be supplied for this attribute. The default is usually "bottom" for the first axis and "left" for the second axis. However, if exterior labelling was requested (see the Labelling attribute) but cannot be produced using these default Edge values, then the default values will be swapped if this enables exterior labelling to be produced.
AST A Library for Handling World Coordinate Systems in Astronomy