Dow theory is invented by charles H Dow in 19th century. It is the directional indicator that understand market trend using price and volume .
In order to reach demand/supply one should follow the below steps
1, open price chart 2 years
2, mark high top , bottom
3, identify sequence
4, volume significance
HB – HT – bullish trend
LT – LB – bearish trend
It derives trend for period of 2 – 3 months