Just like apple pie and cheddar cheese or wayward flies and eccentric scientists (or Jeff Goldblum and anything, really), rap and rock were long banished to live in awesome, but separate, worlds.

It would never work, people said; they were from conflicting families, and rock would never put aside its thrashing guitars long enough to really give rap’s pounding beats a chance. Likewise, what room was there for rock in rap’s life? A rim shot squeezed in for a few milliseconds between scantily clad dancers? That’s no way for a genre to live. It seemed that the two could never reconcile their differences.

Until the listeners illegally did it for them and it was awesome.

Today most mashups are performed live by DJs or circulated through bootlegs and on YouTube, and several are truly stunning.

Consider the case of “Sweet Home Alabama Vs. Country Grammar.” (Actually, look it up now.) What do you get when you cross Lynyrd Skynyrd’s rollicking Southern rock riff with Nelly’s feel-good lyrics about getting high and rolling around St. Louis? If you said anything except the greatest song of all time, then you’re wrong. This is the kind of cooperation that could have prevented a civil war!

And hard as it is to believe that rap and synthpop could be combined to create a dance song greater than the sum of its components, it is achieved in “Sweet Dreams Without Me” with Eminem and The Eurythmics. Epic beats, check. Get-you-out-of-your-seat flow, check. Finally we know what was holding D12 back that whole time. If Kuniva had spent less time being useless and wearing false teeth in the background and had instead been a moaning Scottish woman with computer access, they really could have turned that band around, and maybe Proof would still be alive.

There are even occasionally some mashups that make a big splash and are released on the original artists! What a concept! Arguably the best was Aerosmith and Run-DMC’s “Walk This Way” which came at a turning point in the careers of all stars involved. They single-handedly saved those who created it from the demons holding them captive, be they drug addiction (Aerosmith), obscurity (Run-DMC) or shoelaces (Adidas).

Linkin Park and Jay-Z also released a mashup album titled “Collision Course.” The EP reached No. 1 on the Billboard Charts and won a Grammy, beating out other nominees such as Missy Elliott, Lil Wayne and Kanye West, proving that rap-rock mashups are better than Kanye West.

Jay-Z was also famously sampled by Danger Mouse, a stormtrooper, superhero, nadsat veck and occasional DJ best known as the thinner, taller and more afroed half of Gnarls Barkley. A cappella tracks from Hova’s Black Album were mixed with the music of the Beatles’ White Album to make the mashup masterpiece Grey Album. If you’ve never heard “What More Can I Say” over “While My Guitar Gently Weeps,” then there are deep aspects of human nature that you will never understand.

A slightly less fruitful Beatles mashup that still deserves attention is “Come Closer Together,” blending the Fab Four with Nine Inch Nails. Whoever thought to incorporate the creepy, bestiality-evoking Trent Reznor lyrics into the blissfully ridiculous Beatles tune deserves a medal. At its climax, the song mashes choruses. I’ll give you a second to think about that one. (Hint: the title of the song is “Come Closer Together”). We’re not allowed to print the lyrics, but the juxtaposition of NIN’s gravelly, industrial timbre with the bubbly Beatles is the height of mash.

There are a few other Internet mashups worthy of note that will make you wonder why producers don’t have the good sense just to release the stuff. “Dirt of Your Symphony” is yet another Jay-Z track. (Is there anybody who hasn’t sampled this guy?) If you thought Jay-Z was epic before, give him some backing violins and contrast him with a whiny British man. What makes The Verve seem sensitive to the point of lily-liveredness grants Jay a gravity of which mere sampling so frequently falls short.

And let’s not forget the Travis Barker take on “Crank That.” In his apparent epileptic fit, Barker manages almost enough cymbal crashes to drown out Soulja Boy completely. In “Rick Roll While It’s Hot,” Rick Astley gets to feign the experience of finally having talent, girls and black friends.

While technically not a proper mashup, one final endeavor deserves recognition. I am talking, of course, about “Punk Goes Crunk,” released last April to the joy of literally dozens of people…connected to the project. On the album, you can hear such allegedly punk outfits as Say Anything and New Found Glory rip into rap staples such as “California Love” and “Nothin’ But a ‘G’ Thang” and even, (are you ready for this?), “Men in Black,” originally made infamous by Will Smith, now passing the torch onto Forever the Sickest Kids.

However, the real stand-outs here are The Devil Wears Prada, performing everyone’s favorite middle-school jam, “Still Fly.” If I had to pick, I’d say that it was the metalcore Christians’ particular performance of the following lines in hardcore growling that won me over: “Have you ever seen the crocodile seats in the truck?/ Turn around and sit it down and let em’ bite ya butt/ See, the steering wheel is Fendi, dashboard Armani/ With your baby momma playa is where u can find me.”