[secondcoming]

Symbian was the core OS, phone manufacturers build the GUI on top of it.

Series 60 was the dominant Nokia UI at the time, but then that received a shake-up with Belle?

Fun fact: Until Nokia bought them, Symbian devs never got actually see any phones that were being built, unless you worked in a specific team that had access restrictions to even enter.

[jampekka]

Symbian^3 (Anna and Belle) introduced Qt for the strategy for smooth transition from Symbian to MeeGo. This was killed to go all in on Windows Phone.