Historical Rules · 5 October 2018
Rule 202.3d
Official source text from the 5 October 2018 Rules release
The converted mana cost of a split card not on the stack or of a fused split spell on the stack is determined from the combined mana costs of its halves. Otherwise, while a split card is on the stack, the converted mana cost of the spell is determined by the mana cost of the half that was chosen to be cast. See rule 708, “Split Cards.”