Dow Theory

Dow Theory holds that any lasting rally to new highs in the Dow Jones Industrial Average must be accompanied by a new high in the Dow Jones Transportation Average - the 20-stock index that tracks some of the largest U.S. airlines, railroads and trucking companies. When the transport average lags behind, it can presage broader stock declines. 

Driebusch, Corrie. "Dow Theory Hints at Fall for Stocks." The Wall Street Journal., Monday, June 8, 2015, C1, Accessed on June 8, 2015, http://www.wsj.com/articles/dow-theory-has-investors-skittish-on-markets-next-leg-1433713438