It offers a way to understand market trends and price movements, forming the foundation for many modern technical analysis ideas. 

Dow Theory states that market prices move in predictable trends. By examining these trends, investors can learn about market behaviour and make better investment choices. 

The main ideas of Dow Theory include the concept that markets operate in different types of trends: 
- Primary Trend: A long-term trend lasting months or years that shows overall upward or downward movement. 
- Secondary Trend: Short-term corrections within the primary trend, lasting weeks to months, which partially retrace the main trend before continuing. 
- Minor Trend: Daily variations that happen within secondary trends, usually considered minor and insignificant. 

Furthermore, Dow Theory outlines three phases of market trends: 
- Accumulation Phase: Smart investors begin buying or selling based on their insights while the general public remains uncertain. 
- Public Participation Phase: More investors enter as the trend becomes known, leading to significant price changes. 
- Distribution Phase: Informed investors sell their stock, suggesting the trend may be ending and could reverse. 

Another key tenet is that the market accounts for all information. Dow Theory believes that all known factors, such as economic data and investor sentiment, are already included in stock prices, and price movements are the best indicators of market conditions. 

Volume is crucial in confirming market trends. In an uptrend, volume should rise as prices increase and fall during pullbacks. In a downtrend, volume should rise as prices decrease and decline during temporary increases. A decline in volume amidst price changes may indicate weakness. 

Dow Theory claims that a trend continues until there are clear signs of reversal. Signs include breaks of key support or resistance, differences between price and volume movements, or indexes not confirming each other’s movements. Investors should be careful and not rush to predict reversals. 

Even after many years, Dow Theory remains relevant in today’s financial markets. It underpins various technical analysis methods and helps investors spot long-term trends and manage risk. Its principles apply not only to stocks but also to commodities, forex, and cryptocurrencies, showing its lasting importance. 

In summary, Dow Theory is a vital framework for analyzing market trends and price movements. Its concepts of identifying trends, confirming movements, and analyzing volume still shape modern technical analysis, providing valuable insights for investors’ decision-making.