The letter Q goes back to the beginning of Infiniti in the late 1980s. The very first car they ever produced was the Q45 full-size sedan. The 45 in the name indicated that the car featured a 4.5 L V8 engine. Before the universal Q change, the numbers always indicated the engine size, such as the G37 having a 3.7 L engine. As for the letters, they determined which kind of car it was. Gs were compact cars, EXs and FXs were crossover SUVs, and Ms were mid-size sedans. If a car had an X in the name, that signaled that it was a larger vehicle.
Then, the 2013 name change occurred, throwing nearly every naming convention out of the window. The only one that remained was the use of the X, where a Q would indicate a sedan and a QX would indicate an SUV. Once Infiniti brought in the legacy letter, the numbers no longer correlated to the engines. Instead, they were to signal the evolution of the car. The Q50 was the next step after the original Q45.
50 is now the base level for where Infiniti vehicles start. You have the QX50, followed by the 55, 60, and 80. Vehicles like the Q60 and QX70 have been discontinued, which accounts for the number gaps in the names. Switching their names entirely to this scheme simplifies and standardizes everything. You just have to keep the numbers straight.
