In volages chess, units change color (and then side) the
1st time they move from a dark to a light square or vice versa. After
that, they become normal units. The volages rule does not apply to Kings.
"Volage" is French for flighty or inconstant. This fairy genre was invented
by A. Davaine in 1969.
Retro-volages chess is just volages chess with retrograde analysis.
Here, when we say "the 1st time a unit changes color of square" we
mean the 1st time since the starting position. So that all present units
are assumed to be volages, except if retro-analysis proves they necessarily
already changed color once.
In positions where retro-analysis proves that out of several units,
at most one is still volage, the usual convention is as with
mutually exclusive castling rights,
i.e. the first unit which changes color of square changes side, thereby
depriving the other units of their volage potential and turning them
into normal units.
Here is a delightful example :
Rex Multiplex, 1983
4+3. (Retro-Volages) Mate in 2