Historical Rules · 25 August 2017
Rule 711.4b
Official source text from the 25 August 2017 Rules release
While a double-faced permanent’s back face is up, it has only the characteristics of its back face. However, its converted mana cost is calculated using the mana cost of its front face. This is a change from previous rules. If a permanent is copying the back face of a double-faced card (even if the card representing that copy is itself a double-faced card), the converted mana cost of that permanent is 0.