The last four seasons of Netflix's House of Cards have wavered between shocking and silly, campy and sinister, good and bad. But the show has always had one big, scenery-chewing constant: Kevin Spacey's Southern accent.

Spacey's accent is as crucial to his character, the diabolical Frank Underwood, as venom is to a cobra. The lines Spacey delivers as the conniving congressman (now president) are some of the most ridiculous in television history — "I've always loathed the necessity of sleep. Like death, it puts even the most powerful men on their backs." But they're only enhanced by Spacey's slow-cooked, bourbon-glazed take on the South Carolina drawl.

But is Spacey's accent accurate at all? We talked to a couple of linguists who specialize in American dialects, and found that, well, the accent is just as trustworthy as the maniacal lawmaker himself.

Kevin Spacey's Southern accent is passable. But true Southerners will hear a difference.

"I suspect that some Southerners might recognize that he doesn't sound quite right," Dr. Erik Thomas, a professor of linguistics at North Carolina State University told me. Thomas has studied Southern American English since his undergraduate years and is considered one of the pre-eminent experts on Southern accents. "The average person might not hear the difference," he added.

What Thomas and linguists listen for are "features." Different regions of America have different ways of pronouncing different parts of words. For example, the way someone says the word "home" can sound different depending on how someone's tongue is positioned when they're saying it, and the physical location in the mouth where the sound is coming from.

In the South, some parts of the west, and Baltimore, you may hear something closer to "hewm" (especially if you saw the last season of American Horror Story) because people are pronouncing the vowel sound closer to the front of their mouths. That's a feature.

"It's clear that Kevin Spacey is targeting Southern English," Kara Becker, an assistant professor of linguistics at Reed College told me. She also explained that while Spacey is hitting certain features very well, some distinct sounds are being ignored.

"I would expect to hear more of the Southern Vowel Shift in his speech, as well as back vowel fronting [like hewm mentioned above]. But there are a lot of features of Southern English, and so it's clear he's targeted some salient features to focus on in his accent," she added.

"Not too surprisingly to me, Spacey's impersonation focuses on a few stereotypical features (especially non-rhoticity and glide weakening of /ai/) and misses various other features, notably the PIN/PEN merger but also various other things, such as the exact quality of the /ei/ and 'open o' vowels and the pronunciation of the word want," Thomas added.

Did that make no sense? Don't worry. It will. Read on.

Frank Underwood sounds like a Southerner born before World War II

There are a couple of distinct features that make Underwood "sound" Southern to some people. One of these traits is his "R-lessness," which an expert would call his "non-rhoticity."

"This is when r at the ends of syllables is pronounced like a vowel or deleted, so that car and cars sound like cah and cahs," Becker explained to me. You can hear it in the way Underwood says words like "uninformed" and "careless."

This is one of the features that Thomas considers a stereotypical Southern feature. It sounds the way people think Southerners sound, but it's actually a feature that is disappearing in Southern speech.

"R-lessness was a traditional Southern feature — although not everywhere in the South — it was most prevalent in areas where the plantation culture had pre-dominated at one time before the Civil War," Thomas told me, explaining that non-rhoticity had once been considered prestigious.

"What's happened since World War II is that white Southerners have really abandoned the R-lessness — they've gone to saying their Rs all the time, and they've done it pretty quickly too," he added.

Thomas explained that the people who use this feature are older, and that Underwood would have to be born before WWII in order for this accent to make sense. Underwood, of course, is supposed to to be around 55 years old.

A real-life example of that shift in the Rs, is South Carolina Senator Lindsey Graham. Graham is 59 years old — a bit older than Underwood, and born 10 years after WWII. He pronounces all of his Rs. In linguistic speak, he is completely rhotic:

Another distinct, and possibly outdated feature that Underwood is using is pronouncing is reversing his "wh" in words like whip and whale, which turn into hwip and hwale.

"Of note, though, is Lindsey Graham does this too — only older speakers should do this," Becker noted. "These aren't used by younger Southerners, but a speaker of his age from South Carolina could use them sometimes."

Frank Underwood is missing his "Open-Os"

Another feature that Spacey doesn't quite get right about his Southern accent are the "open-o's" in his speech. They're missing.

"The open-o is what's often referred to as the 'aw' sound," Thomas said, explaining that Southerners might use the sound when pronouncing talk more like tawwk. "Spacey doesn't do that at all. You'd hear it in words like off and caught."

Thomas pointed out that in the Graham video (above), you can hear him employ the open-o sounds.

Why Frank Underwood sounds Southern? Blame monophthongization.

Monophthongization is a mouthful and it's one of the things, along with non-rhoticity, that Spacey has honed in on. It's the biggest reason why people "sound" Southern.

"This is the defining feature of the South, a huge dialect area stretching from North Carolina to Texas (east to west) and from the Ohio River down into Florida (north to south)," Becker explained to me.

"In words like my and time, Southerners produce the vowel /ai/ — normally a diphthong (a vowel with two parts) — as a monophthong (a vowel with one part), so that these words sound like mah and tahm. Frank Underwood uses this feature in words like why, prize, beside, eyes, terrified."

Frank Underwood's accent is intriguing because the traditional Southern accent is disappearing.

Frank Underwood is fascinating, because it seems like there's no one else who sounds like him on television. That's becoming more and more true in the real world too. Thomas explained to me that traditional Southern accents are slowly becoming sounds you only find in rural areas.

"It depends on what state you're talking about," he said, explaining when you start to see the Southern accent slowly lose its grip. "In Texas, you look at people born in the 1960s. In North Carolina, a decade or two later than that — the 70s and 80s."

Thomas explains that in those years, those states saw huge growth and an influx of people who weren't from the South moving to the states' bigger cities. That growth created new suburban communities that sometimes didn't include locals.

"It's sort of like starting over from scratch," Thomas said. "Kids growing up in those communities weren't learning their English from traditional local people, or kids who were from traditional local families, but people from outside."

But there's also a cultural aspect to this too. There continues to be something of a stigma against Southern accents.

"In situations like that, where you have a lot of Southerners and non-Southerners, and kids are learning their English, which forms are they going to choose? The prestigious one, or the one they're going to make fun of?" Thomas said. "That's what's going on in the South these days."