When it comes to fighter jets, the modern world is currently amid a transition. The combat-proven fighters we’ve grown to know and love over the years are reaching the ends of their operational lifespans, and while update programs are underway for many of them, technological strides made in the years since these platforms first took to the skies have finally made it feasible to field a whole new generation of aircraft, more competent and capable than any that came before them.

Or at least, that’s the idea.

These new aircraft fall into the “fifth-generation” of fighter classes fielded by the world’s top militaries. However, what exactly sets them apart from aircraft like the F/A-18 Super Hornet or the F-15 Strike Eagle? At what point does a new design stop being an advancement of a previous generation and become a new generation all it’s own?

Well, the answer is a bit more complicated than you might think. Just like the lines between Generation X, millennial, and whatever other generational names get slapped on folks of certain age groups, the edges of where one generation ends and the next begins start to get blurry the closer they are together. There’s no doubt that the F-16 Fighting Falcon is a fourth generation fighter, but there’s a reasonable debate to be levied about China’s current iteration J-20 despite its general acceptance as a next-gen aircraft. There are some generally accepted conditions an aircraft must meet to be considered a part of the “5th generation,” but what if a plane meets most of them? Or promises to meet all of them later in its operational lifespan?