Have you ever wondered why some gas tanks are on the right side of the car, while others are on the left? It’s the kind of mystery that hits you as you’re waiting in line to get gas. After all, at that point there’s not much else to look at but wall-to-wall cars and car parts.

So what is the answer to the question? Why aren’t gas tanks always on the same side of every car? It seems that an engineer is free to put the gas tank door wherever it makes the most sense: whatever makes the car easier to package. According to Steve Yaeger of Nissan, the placement of the gas tank has to do with “fuel tank design, location, and underbody packaging.”

Yaeger explains that the structures and components located on the undersides of vehicles can vary from car to car, presenting obstacles that determine where the fuel tank door might best be placed.

Americans Like Their Gas Tank Doors on the Left

Americans tend to like their fuel tank doors on the left side of their cars, because this makes it easier to get the left fender up close and personal to the fuel pump. It goes to follow then, that drivers in countries where they drive on the left side of the road, are going to prefer that gas tank door on the right. Some experts have suggested that the position of the gas tank door varies because if all cars had them on the same side, half of all gas station pumps would sit unused and we’d spend even more time waiting to fill up our tanks.

Here’s a piece of interesting trivia: if you can’t remember on which side your fuel tank door is located, you only need to look at the diamond-shaped arrow located on your dashboard’s fuel gauge. That arrow is always dependable, pointing to the side of the car on which your gas tank door is located. Take a glance at that arrow before you choose a gas pump, so you don’t have to red in the face as you return to your car to drive to a different pump.