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+PrBoom compatibility options
+============================
+
+Some of this is a bit dry. You might want to skip to the bottom and read the
+examples.
+
+Internally, PrBoom maintains a "compatibility level" which it uses to
+determine how to behave in various circumstances. The supported levels are
+currently:
+
+Number	Name			Description
+0	doom_12_compatibility	Emulate Doom v1.2 game details where possible
+1	doom_1666_compatibility	Behave like Doom v1.666 as far as possible
+2	doom2_19_compatibility	As compatible as possible for playing original 
+				Doom demos
+3	ultdoom_compatibility	Ultimate Doom made a few changes
+4	finaldoom_compatibility	Compatible with Final Doom & Doom95 - needed
+				for some demos recorded with Final Doom doom2.exe
+5 	dosdoom_compatibility	Emulate early dosdoom versions
+				(some TASdoom demos work with this)
+6	tasdoom_compatibility	unused
+7 	boom_compatibility_compatibility	Emulates Boom's compatibility mode
+8	boom_201_compatibility	Emulates Boom v2.01
+9	boom_202_compatibility	Emulates Boom v2.02
+10	lxdoom_1_compatibility	Emulates LxDoom v1.4.x
+11	mbf_compatibility	Emulates MBF
+12	prboom_1_compatibility	Emulates PrBoom v2.03beta
+13	prboom_2_compatibility	PrBoom v2.1.0
+14	prboom_3_compatibility	PrBoom v2.1.1-2.2.6
+15	prboom_4_compatibility	PrBoom v2.3.x
+16	prboom_5_compatibility	PrBoom v2.4.0
+17	prboom_6_compatibility	Current PrBoom
+
+You can cycle through the compatibility levels in the game using the TNTCOMP
+cheat. There's also the default_compatibility_level config file option, and
+the -complevel command line parameter.
+
+The numbers are subject to change between versions, so if you're doing
+elaborate stuff with these things you're advised to check this file each
+time you upgrade. Most people should just leave default_compatibility_level
+set to -1 in the config file, which means "use most recent" which has most
+features, most bug fixes, etc.
+
+But some people want to test the behaviour of levels with older versions, and
+will find it helpful to not have to load a dozen games to do so. Some people
+like me play a lot of old levels, so need to be able to enable compatibility
+with some old bugs. And some people may want to record demos for older game
+versions.
+
+When you play a demo, PrBoom sets the compatibility level and settings 
+automatically. When you load a savegame, the settings are restored.
+
+When you start a new game, the compatibility level is got from (in order of
+preference):
+- -complevel parameter
+- default_compatibility_level config file param if not -1
+- set to the most recent otherwise
+
+If the compatibility level is MBF or better, the detailed compatibility
+settings are read from the comp_* config file options, as in MBF. Otherwise,
+the settings are put to the defaults for that game version.
+
+You can adjust the compatibility settings during play from the menus
+(Options->Setup->Doom Compatibility). 
+
+The sort of people interested in these things will already know the MBF
+options, so I'll just list the changes:
+- comp_floors now also causes the original Doom behaviour of objects stuck in
+a ceiling preventing the floor below lowering.
+- new option comp_moveblock enables the old movement clipping bug which
+allows things to go through walls sometimes (mancubus shots, and key
+grabbing tricks)
+
+PrBoom can also record old demos with some success (demo players see the
+comp_moveblock note above):
+
+prboom -complevel 2 -record test
+
+records a doom v1.9 compatible demo
+
+prboom -complevel 9 -record whatever
+
+records a Boom (v2.02) demo
+
+prboom -complevel 11 -record blah
+
+records an MBF demo.
+
+Of course, demo recording is no more reliable than demo playback; original
+Doom demos will usually but not always work. MBF support should be perfect.
+
+- Colin <cph@moria.org.uk>
+