The TED spread is the difference between the interest rates on inter-bank loans and short-term U.S. government debt ("T-bills").
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The TED spread is an indicator of perceived credit risk in the general economy[1]. This is because T-bills are considered risk-free while LIBOR reflects the credit risk of lending to commercial banks. When the TED spread increases, that is a sign that lenders believe the risk of default on inter-bank loans (also known as counterparty risk) is increasing.
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TED is an acronym formed from T-Bill and ED, the ticker symbol for the Eurodollar futures contract. The size of the spread is usually denominated in basis points (bps). For example, if the T-Bill rate is 5.10% and ED trades at 5.50%, the TED spread is 40 bps. The TED spread fluctuates over time, but historically has often remained within the range of 10 and 50 bps (0.1% and 0.5%), until 2007. A rising TED spread often presages a downturn in the U.S. stock market, as it indicates that liquidity is being withdrawn.

During 2007, the subprime mortgage crisis ballooned the TED spread to a region of 150-200 bps. On September 17, 2008, the record set after the Black Monday crash of 1987 was broken as the TED spread exceeded 300 bps[3]. Some higher readings for the spread were due to inability to obtain accurate LIBOR rates in the absence of a liquid unsecured lending market[4]. On October 9 2008, the TED spread reached another new high of 423 bps.