As a concept, mark-to-market is simple to understand, but it is a burden to deal with on a daily basis. In the past, most banks had rarely if ever bothered putting a dollar amount on illiquid investments, such as real estate or mortgages, that they planned to keep. Most banks valued their illiquid investments simply at the price they paid for them, rather than venture to estimate what they might be worth on any given day. If they later sold them for more than they paid for them, they made a profit; if they sold them for less, they recorded a loss. But in 2007 that straightforward equation
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