The release of Kendrick Lamar's "The Heart Part 4" on Thursday night sent the rap world into a frenzy. Between trying to figure out which rapper K. Dot was dissing, making memes, and dueling with the Bey Hive, rap fans were kept plenty busy. 

But it was the "Part 4" in the song's title that also received a lot of interest. And, as Lamar stans knew and casual fans quickly discovered, there are indeed three previous parts in the series, all from very different times in the rapper's career. Part 1 was a loosie, from several months before the release of Overly Dedicated.

Part 2, the only one of the series to appear on a proper project, was the opening track on O.D. 

Part 3 was recorded and rush-released days before Good Kid, M.A.A.D. City came out.

And Part 4 was released just in advance of, well, whatever is going to happen on April 7. 

 So the question is, what makes them a series? Other multiple-parters in hip-hop have had recurring characters (see EPMD's six-part "Jane" series); some kind of consistent sound or aesthetic (Rick Ross' five-part "Maybach Music"); a continuing story (Jay Z's "Coming Of Age" and its sequel); a similar approach (Outkast's appropriately-titled "Da Art of Storytellin'" in all its incarnations); or the same beat (Dr. Dre's "The Watcher" and its follow-up).

At first glance, the "Heart" series would appear to not have any of those things in common. But the songs do have similarities embedded deep in their DNA—similarities that might demonstrate why they're grouped together. 

First of all, the songs all paint a vivid picture of where Kendrick is at that particular moment—his concerns, his fears, his obsessions. When talking about the second part, he said, "“I think that’s probably one my most heartfelt joints; [that's the] reason why I called it that." That quote could apply equally to the other three parts of the series.

A look at the first lines of each of the parts will reveal a lot about how Kendrick sees himself at that particular moment in time. In Part 1, he's "a lil' Compton nigga" who's "back with a vengeance." But seven years later at the top of Part 4, he's a "hip-hop rhyme savior" with "legendary status" and $30 million in the bank.

There are also structural similarities. The songs contain long verses (most of them are just one long verse), and barely any hint of a hook. By the time Part 4 comes around, there are several beat switches as well. Part 3, the only one to feature rappers other than Kendrick, has Jay Rock and Ab-Soul coming in for verses of odd lengths. So the songs cohere not because of what they are, but because of what they're not: a conventional song, with a predictable, repeatable structure—16-bar verse, 8-bar hook, repeat ad nauseam.

Finally, a close look at the Heart series will reveal themes that keep popping up. You can find these throughout Kendrick's catalog, for sure. But the fact that they appear over and over again in his most unguarded, heartfelt moments shows that these ideas are key to who he is. Let's examine them a bit.

The Bible:

Biblical references abound in this series. In Part 1, he's both praying and preying while reading Corinthians. By Part 2, he's backing off a bit: "I ain't seen too many churches/Or know them Testament verses," he admits. And in Part 3, he's going to trial "with a Bible and a rifle."

Jay Z:

While Kendrick is hardly the only Hov obsessive in TDE, his relationship with Jay, or the idea of Jay, has always been fascinating. This is definitely true throughout the Heart series. Kendrick quotes Jay in Part 2. In Part 3, he admits that he modeled his early career on Roc-a-fella ("Me and Dave inside the garage/And thought we was Jay and Dame, that's the lane we tried to drive"). And in the newest song, K. Dot not only interpolates a few Jay lyrics, he also directly references Hov's recent election to the Songwriters Hall of Fame.

Death:

The specter of death, whether his own or someone else's, hangs over Kendrick's head throughout the Heart saga. "Niggas dying, motherfuck a double entendre," he seethes in Part 2. By Part 3, the pressure of being seen as "Pac reincarnated" (a theme he would revisit a few years down the line) is almost too much. "That's enough pressure to live your whole life sedated," he raps. "Find the tallest building in Vegas and jump off it/But I could never rewrite history in a coffin." In Part 4, he's imagining himself already dead.

Compton:

Kendrick's career is shaped by his hometown—its people, its culture, and, not least of all, its hip-hop legacy (shout-outs to Compton rapper Glasses Malone are another amusing little theme of the Heart series). And you can see Lamar's relationship to the town change as the years go on. In Part 1, he claims it—literally the first thing he says is that he's a "lil' Compton nigga." By Part 2, he's thinking bigger: "I pray these bars get farther than Compton." On Part 3, on the verge of releasing his Dr. Dre-sanctioned debut album, he's wondering, "How can I make an example for this generation of Compton?" And the international superstar of Part 4 is demonstrating that he hasn't forgotten his roots by mentioning that he owns a condo there. 

There is a whole lot more in the Heart series. It is a fascinating series of snapshots of the career of arguably this generation's most important artist, and we can all be glad that Part 4 is bringing renewed attention to this unusual and unguarded corner of Kendrick's output.