Dow theory was developed by Charles Dow and it basically studies about the market trend based on the demand and supply.In this theory the top side belongs to supplier zone and the lower side to buyer zone.We need line charts and at least 2 years of data to analyse the trends and it is important to identify the higher bottom -higher top, and lower top-lower bottom, where HB-HT is bullish trend and this is a buy signal and LT-LB is bearish trend and this is a sell signal.This theory is not suitable for short term investments but still it remains on top of all analysis because of the basic concept of demand and supply.Its important to know the support and resistance levels.