This is one concept in technical analysis to identify trend of the stock.Line chart is used in dow theory. Minimum period is 2 years.Dow theory is ok for long term analysis. But, to predict short term trend, entry exit levels is not possible by using Dow theory. However, till date, this is one of the greatest one to identify trend of the market for long term. Most of traders are deciding long position basis dow theory.when price of stock breaks above recent higher top, we could take long position. In opposite, when price breaks below recent lower bottom, sell position can be taken.Support is level where buyers are aggressive and Resistance is level where sellers are aggressive.