Historical Rules · 1 October 2012
Rule 202.1
Official source text from the 1 October 2012 Rules release
A card's mana cost is indicated by mana symbols near the top of the card. (See rule 107.4.) On most cards, these symbols are printed in the upper right corner. Some cards from the _Future Sight_ set have alternate frames in which the mana symbols appear to the left of the illustration.