There are many ways for the characteristics of a permanent in play to have its characteristics changed. Double-faced cards are just a new way of changing more than one of these characteristics at one time, and in a somewhat dramatic way. While on the battlefield, these cards have the chance to turn over completely and change all of their base characteristics.

However, these changes only apply while in play. In any other zone–library, graveyard, hand or exile–these cards are treated as only having the characteristics of their face-up (“day”) side. Effects that look at, use or try to match the characteristics of a card will only ‘see’ the face-up versions of these cards.

For example: while in play, Gatstaf Howler has a converted mana cost of zero. This is because the back side of a double-faced card has no mana cost, and therefore its converted mana cost is zero. However, when in the graveyard, its characteristics are those of its face-up side. It does not matter that it left play as a Gatstaf Howler; in the graveyard, the only characteristics that matter are those of the Gatstaf Shepherd side. Because of this, if you were to target this card with Postmortem Lunge, you would need to pay 2B or 2 mana and 2 life in order to return it to the battlefield. Also, it will be returned to play as a Gatstaf Shepherd, not a Gatstaf Howler.