Historical Rules · 5 October 2018
Rule 711.4b
Official source text from the 5 October 2018 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. 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.