Tesla has recently been releasing videos going behind the scenes on some of their most famous features. The latest of these videos details the inner workings of one of their most popular gimmicks: the Model X’s Falcon Wing doors.

It’s always interesting to see an early-stage prototype of a technology that’s made it to the mainstream, and the Falcon Wing doors are no different. As we can see in the video, the doors that we now associate with the Model X were originally tested on a Model S, which makes sense considering the SUV hadn’t come out yet. But rather than being integrated into the sedan’s design, the control module was a clunky addition that was retrofitted to the roof. Additionally, it looks as though some of the mounting points intrude into the rear seating area, which goes to show how far from production this prototype was.

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One of the advantages the Falcon Wing doors have over something like a gullwing door is that they require less space to open, both horizontally and vertically. This is due to a second hinge above the window that allows the doors to fold themselves tighter as they raise and lower, as well as loads of sensors to make sure they don’t run into anything as they move. Compare that to traditional gullwing doors, which only have a single hinge on the roof, require more space to open, and don’t feature any sort of obstacle avoidance measures.

This prototype has those secondary hinges, but they’re barely putting in any work when compared to the articulation we see on the production doors. And speaking of said production doors, Tesla also included a clip of them in action so you can take a back-to-back look at how far the technology has come.