43.16 groff

troff (43.12) was originally designed for a now-obsolete typesetter. One of the main limitations of this typesetter was that it could only use a total of four fonts on the same page - usually the Roman, Bold, and Italic versions of the same typeface, plus a special font that included math characters and various other symbols. It doesn't let you do things like you see on these pages, which typically include seven or eight fonts: the four listed above, plus a completely different typeface for the headings, plus a constant width font (with bold and italic variations) for use in showing scripts and examples.

The original troff was replaced with a device independent version called ditroff (43.15), which addressed these limitations and added a lot of other new features as well. Unfortunately, on UNIX systems prior to System V Release 4, ditroff was unbundled from UNIX and sold as a separate product, so all that some systems offer is the old version.