HFWG Recommendation R1:
On the use of underscores and hypens in FITS keywords and column names(Approved: 1993 Jun 30)
It is strongly recommended that underscores ('_'), rather than hypens ('-'), be used as punctuation-type characters in FITS keyword names and in FITS table column names (i.e. in the value field of the TTYPEn keywords, where n is the column number). It is hoped that this recommendation will eliminate needless debate over which character to use when creating new keyword names and reduce confusion and errors on the part of software developers and users when entering keyword names.
The only exceptions to this recommendation are when: