Welcome to fvwm. Fvwm is a multiple large virtual desktop window manager, originally (a looooong time ago!) derived from twm.
Fvwm is intended to have a small memory footprint but a rich feature set, be extremely customizable and extendible, and have a high degree of Motif mwm compatibility.
ALL VERSIONS OF FVWM PRIOR TO 2.6.X ARE NOT SUPPORTED UPSTREAM.
Users of fvwm prior to 2.6.X can make use of the [./bin/fvwm-convert-2.6.in](fvwm-convert-2.6 utility) to try and have their configuration file converted to a newer syntax. Please see its manpage for more details on how to use it.
Those interested in contributing to FVWM should have a read of the developer documentation.
If you have any questions, concerns, bug reports, enhancement requests, etc., please feel free to send an email to the fvwm mailing list, or open a Github issue.
Have fun!
-- The fvwm-workers