Initially, the TED spread was the difference between the interest rates for three-month U.S. Treasuries contracts and the three-month Eurodollars contract as represented by the London Interbank Offered Rate (LIBOR). However, since the Chicago Mercantile Exchange dropped T-bill futures, the TED spread is now calculated as the difference between the three-month T-bill interest rate and three-month LIBOR.The TED Spread (Wikipedia)
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. Some higher readings for the spread were due to inability to obtain accurate LIBOR rates in the absence of a liquid unsecured lending market. On October 10, 2008, the TED spread reached another new high of 465 basis points. The longterm average of the TED has been 30 basis points.