In a way I see it like this. Lets put the difficulty scale of 1 to 4, EZ=1 NM=2 HD=3 MX=4, for each keymode the MINIMUM of two difficulties must have a difference of 2 on the difficulty scale. For example 1 and 3 which is EZ and HD or 2 and 4 which are NM and MX. If you want to put EZ and MX though I guess you'll have to throw in an NM/HD in there too. If you can map more than 3 for each keymode you're doing, that'd save you a lot of trouble
I've pondered about this rule since I read it for the first time and went "wat".
I've pondered about this rule since I read it for the first time and went "wat".