To a certain extent, everything from OD&D through 3.5e (and quite possibly 5e) are somewhat compatible. Back in my early gaming I was using D&D adventures in my AD&D game without batting an eye.
These days, I've used LL, C&C and S&W adventures and supplements in my ACKS campaign. Most of the conversion work I can do on the fly, but there are times I wish I had spent a few minutes ahead of time making sure things were ready for ACKS.
I've also grabbed some stuff from 3x and Pathfinder with minimal effort in converting, but I suspect the reverse would be more time consuming if not downright frustrating.
So, do you mix and match OSR/OGL rules in your games? Convert on the fly or scratch it out ahead of time?
The Online Campaign, Proposed Again - Let's seriously discuss an online campaign. At present, I have two volunteers, Engelhart and Pandred, who have already agreed to play, both of whom have c...
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