Good afternoon, everyone! Today is Friday, the week is over, and that means it’s time for Letters to Doug, your favorite weekly automotive column where I tackle some of the least pressing automotive concerns of our day.

Letters to Doug is not a unique club that you aren’t allowed to join unless you have a CarMax Range Rover. On the contrary: you, too, can participate by sending me a letter at Letters2Doug@gmail.com, or by tweeting me at @DougDeMuro. And don’t forget: names will be changed to protect the letter writer, so you can ask any type of zany question that you want.

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Anyway: today’s letter comes to us from a reader I’ve named Jim, who’s writing from Los Angeles, California, the beautiful City of Angels Who Are Choking on Smog. Jim writes:

Hi Doug,

I saw a 4 series 4 door the other day here on the streets of Los Angeles. If BMW made the 4 series and thus the M4 to differentiate the sedan and coupe models why the fuck is there a 4 series sedan (Gran Coupe)? The name doesn’t even make sense since a coupe is a 2 door. Please explain this for me.

thanks

Jim

Ah, Jim is referring to BMW’s new naming scheme, wherein the 1 Series became the 2 Series, and the 3 Series Coupe became the 4 Series, and the 7 Series became bolted to the showroom floor the moment the new S-Class went on sale.

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Now, the theory behind this name change was very simple, back when BMW first announced it: two-door cars would have even numbers, so that us consumers could easily divide the vehicle number by the number of doors and not run into any confusing situation where we had one of those pesky remainders. Whereas four-door cars would have odd numbers, because odd numbers are the only other type of numbers, after you’ve assigned all the even numbers to two-door cars.

And this worked fine and well for approximately seven hours until BMW started hacking up the numbering scheme in order to provide for every single one of its automotive segments, including the midsize luxury hatchback segment and the slightly larger luxury hatchback segment and the even larger luxury hatchback segment and the four-wheel drive luxury hatchback segment, and the armadillo with a combustion engine segment.

And now, we have something of a convoluted mess.

There’s the 2 Series, which is a two-door car and therefore fits in line with BMW’s naming scheme.

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There’s the 3 Series, which is a four-door car. But there are also two separate 5-door versions of this vehicle that BMW is hoping we won’t notice. Based on sales figures, I think we’re doing a pretty good job.

Then there’s the 4 Series, which is primarily a coupe or a convertible. But there’s also a four-door version, called the Gran Coupe, which looks exactly like the 3 Series Gran Turismo.

Next up is the 5 Series, which features both a four-door sedan and a 5-door hatchback model called the 5 Series Gran Turismo.

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Then you have the 6 Series, which has roughly the same problem as the 4 Series: it’s primarily a two-door coupe or convertible, but there’s also a four-door version called the Gran Coupe that is purchased by people who get incredible lease deals.

And then, at the top of the range, you have the 7 Series, which is a full-size four-door sedan that is only driven by people who live in states where they’ve banned Tesla dealers.

So to sum up, the situation is this: 2 Series is a two-door. 3 Series is a four- or a five-door. 4 Series is a two- or a four-door. 5 Series is a four- or a five-door. 6 Series is a two- or a four-door. 7 Series is a four-door. So how can this make sense?

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Well, ladies and gentlemen, I have devised a relatively easy way to think about it: even numbers are for sporty vehicles, and odd numbers are for mainstream ones.

Consider it: the 3 Series (odd number) is a sedan, or a hatchback, or a wagon. Practical, buttoned-down, traditional German bodystyles. Whereas the 4 Series (even number) is a cool coupe, or a sporty convertible, or a four-door sedan that retains coupe-like styling and other coupe-like properties, such as the name “Coupe.” Same deal with the 5 Series – a staid, traditional hatchback and a sedan – compared to the 6 Series, which comes as a coupe or a convertible or a sleek, sexy, Gran Coupe that’s technically a sedan, but BMW wants you to think of it as a four-door coupe.

It also works for SUVs. The X3 and the X5 (odd numbers) are the boxy, practical ones, while the X4 and X6 (even numbers) are the “sporty” ones that look like giant wheeled armadillos with no rear headroom. And it even works for BMW’s electric cars: the i3 (odd number) is the practical, city-friendly hatchback, while the i8 (even number) is the cool, futuristic sports car.

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So, Jim, great news: I’ve cracked the challenging, complex code of the modern BMW lineup. Even numbers are for fun, exciting, sporty products, while odd numbers are for the brand’s more practical, traditional models. Thank you very much for your question, and I’m so glad that my vast, unending automotive expertise was able to assist let another letter writer.

Wait, what’s that? They sell a five-door minivan? Well then of course it must have an odd number, owing to its practical, family-friendly body style! Wait, it’s called the 2 Series? Well, shit. Who the fuck is running this car company?

@DougDeMuro is the author of Plays With Cars. He owned an E63 AMG wagon and once tried to evade police at the Tail of the Dragon using a pontoon boat. (It didn’t work.) He worked as a manager for Porsche Cars North America before quitting to become a writer, largely because it meant he no longer had to wear pants. Also, he wrote this entire bio himself in the third person.