Dow theory is a theory which includes the High Tops and High Bottoms for a longer Duration Period.minimum look up period[2 yrs], duration of trend forecasting for Dow Theory?: Line Charts will be used. We can determine Market Overall Trend using Dow Theory by identifying Top’s and Bottom’s. Buy: when higher bottom crosses Previous higher Top Sell: when it breaks Dow theory pattern (Higher Tops and Higher Bottoms) . the previous bottom is Support and the previous top is resistance.