Do accounting rules actually pack such a wallop? For readers not schooled in financial jargon, marking to market is the practice of revaluing an asset quarterly according to the price it would fetch if sold on the open market, regardless of what was actually paid for it. Because the practice allows for no outdated or wishful-thinking valuations, it is a key component of what is known as fair value accounting. And it is at the center of the hottest accounting debate in decades.