BMW and Audi model nomenclature.
For a long long time BMWs came in S / M / L sizes: 3 series, 5 series, and 7 series.  OK, fine, there was no designation for 2 or 4 doors, but whatever.  Same with Audi, using even numbers instead of odd.
Then BMW added the 6 series (2-door 5 series) and the 8 series (2-door 7 series).  Also fine... but the 2-door 3 series was still a 3 series.  WTF?!  This went on for at least almost 2 decades.
Finally, in the early 2010s, they corrected the problem, renaming the 2-door 3 series as the 4 series.  Rejoice!  It makes sense now!  But wait... that joy will be short-lived.
After finally getting it right after decades of being idiots, BMW promptly went back to being idiots within just a few short years, introducing a FOUR-DOOR 4 SERIES, the 4-series Gran Coupe.  They already HAD a series for compact-size 4-doors: THE 3 SERIES!  Are they stupid, or just TRYING to piss me off?  Did a 2-series even ever exist, as the 2-door version of the 1-series?  And don't get me started on the whole concept of a 4-door coupe.  BMW's not alone in that bit of idiocy.
Audi did exactly the same thing with the A4 / A6 / A8.  The A5 was the 2-door A4, we got it.  So why does my boss drive a 4-door A5?
These are Germans, fercryinoutloud.  The very people who pride themselves on having such a rational, system-oriented thinking process.