NBC News has the article Brexit Wiped $2 Trillion Off Markets: Standard & Poors.

NEW YORK — The $2.08 trillion wiped off global equity markets on Friday after Britain voted to leave the European Union was the biggest daily loss ever, according to Standard & Poor’s Dow Jones Indices.

I won’t even wait for the first person to ask who’s got that $2 Trillion.  Nobody started up an incinerator and burned $2 Trillion.  The money, never existed in the first place.  That amount of money was just notional.  In other words, the accountants of the world who calculate the “mark to market” value of all the stock holdings in the world calculated what all the stocks in the world could be sold for if they were all sold for the amount of money that was paid for the last stock trade in each stock.  Anybody who knows anything about markets of any kind knows that this could never happen, yet people seem to act as if their own stock holdings have this fictitious value.

Well, “mark to market” is not irrational if you understand its place.  Valuing an individual’s stock holdings this way gives you a reasonable approximation of what you could get if you sold your holdings at the instant that the calculation was made.  This assumes that your stock holding is small enough that your sale of it would not impact the market.  It also assumes that the price is not fluctuating significantly at the time you would consider selling it.  It is that last assumption that is what really makes “mark to market” only a crude approximation at times of high market volatility.

Owners of large market positions (mutual funds, hedge funds, pension funds, and the very wealthy, e.g.) are fully aware that they cannot make large sales at one time and get the full, current price the market is paying for small lots of stock.