A "smash-up," also known as a mash-up, bootleg, blend, cut-up, crossover, or powermix, is a song or composition created when a producer blends a pair (or more) of previously existing songs, generally by combining the vocal track from one song with the instrumental track of another. Smash-ups have been around a long time but became a renewed cultural phenomenon with 2004's Grey Album, a smash-up by Danger Mouse of Jay-Z's Black Album and the Beatles' White Album.
Mash-ups are not confined to music, of course. Videogames have smash-ups (often called "mods") and so do tabletop RPGs. Smashing up RPGs has been defined as "replacing major rules components with components from different games" or "taking two games in your collection and mixing them together." However you define it, it's a lot of fun; indeed, one of my favorite pastimes as a gamemaster is blending systems and settings to create something unique.
The amusing thread on RPG.net linked above takes a humorous approach to smash-ups, leading to games like Skyrealms of Tekumel (combining obscure fantasy games Empire of the Petal Throne and Skyrealms of Jorune) to create a "world so weird no one goes there at all." Obviously the writers are doing it just for giggles, but the key to any smash-up is to notice how two systems or settings fit together. I call this "finding the hook," and it's the first step in a three-part process:
- Hook - You notice a rule mechanic or setting element from one game that somehow fits into the framework of the rules or setting of another game
- Blend - You adapt the rule mechanic or setting element from the first game into the second game
- Double Check - You review the adjacent rules or setting elements to make sure the smashed-up game is still coherent
It's easiest to explain in practice, so let's start smashing.
Let's begin with Steve Jackson's Car Wars, a widely played simulation of automobile combat in a post-apocalyptic future. While Car Wars is mostly about car combat, it features the skeletons of a role-playing system: Characters are rated with skills like "Gunner" and "Mechanic" with ratings from 0 to +5, while task resolution is resolved with a roll of 2d6 plus the skill level to meet a difficulty number. For instance, to hit with a machinegun requires a roll of 7 or greater, with the character's Gunner skill level added to the total of the dice.
As it happens, both this skill system and this task resolution are precisely mirrored in GDW's Classic Traveller, the influential science-fiction RPG. That's right - in Classic Traveller, your PC can die during character generation. In Classic Traveller, characters are rated with skills like "Gunner" and "Mechanic" with ratings from 0 to +5, and task resolution is resolved with a roll of 2d6 plus the skill level to meet a difficulty number. This is our "hook" - we've noticed that the core mechanics of the two systems are identical.
What makes this interesting is that Car Wars offers virtually no character development around this system; on the other hand, Classic Traveller is famous for its character generation system, in which characters must go through a career path that can have them enter military, gain promotions and commendations, earn rewards such as weapons, armor, or starship, and even die in the line of duty! The obvious mash-up, then would be to adapt the Classic Traveller career path system to Car Wars, replacing e.g. Space Marines with Autoduelists, Scouts with Outriders, and rewards like spaceships with automobiles. That's our "blend" - we're adapting the character generation from Traveller into Car Wars. We cleverly call this new system Driver.
Once the blend is written, the "double check" would be to assess what this would do to the adjacent rules. We note that Classic Traveller produces characters with on average 4 to 6 skills at +1 while Car Wars starts characters with 3 skills at +0. So our smash-up is going to create characters that are more competent than starting Car Wars characters. We make a note that to challenge characters created using our Driver system we'll need to make sure major NPCs are rolled up using a similar system.
As it happens, someone has actually done this smash-up; you can find it implemented at The Daemon Mechanic.