Ford sells two different cars called "Focus" in China, the "New Focus" shown here and an older model called "Classic Focus". But it counts the sales as one, which has led to controversy. Photo credit: Ford Motor.

Ford (F 0.08%) this week said that its Focus was the world's best-selling automotive "nameplate" in the first half of 2013. If "nameplate" sounds like a carefully chosen word, it was: Ford's claim was immediately countered by a protest from Toyota (TM 1.07%), which disputes Ford's math and says its Corolla is the world's real best-seller.

Who's right? As Motley Fool contributor John Rosevear explains in this short video, it depends on how you define "Focus" and "Corolla," and getting to the bottom of this will be much more complicated than you might think. 

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