First, some background context: I've played a lot of D&D over the years. I've always played about four traveler games, which in comparison is not that much, but I'm totally familiar with the classic traveler system (and, once, with T20; and this time with Mongoose Traveler).
And getting ready now to run a Trav campaign, which is definitely just as old-school a thing to do as running an OSR D&D game, something struck me.
D&D was the work of genius that started it all, a ramshackle mess at first and never totally getting itself organized or streamlined, and representing the spirit of this kind of make-it-up-as-you-go school of design.
D&D came out in 1974.
Traveler came out in 1977.
So ready? Here it is: almost everything important in mechanical game design for RPGs was done by 1977.
Traveler is a streamlined masterpiece of a single-mechanic system. Its simplicity and genius of design is almost a work of art in and of itself.
(even the style looks like it belongs in some kind of German School of Design)
But here's the really important part: I'm NOT saying what I said above as though D&D was the sloppy start and Trav the perfect end-point. What I am saying is that those two games TOGETHER represent everything that is important in design: the DIY aesthetic and smooth sleek careful design on the other.
Pretty much everything that came after? Redundant, or secondary. The core concepts that actually matter are all right there, in two of the earliest games of our hobby.
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