Thomas Demark's lines, or TD lines, are a popular technical analysis tool that provides a simple and objective way to identify possible turning points in a market. TD lines are based on a series of counting rules that help determine the beginning and end of a trend, and they can be used on various time frames, making them suitable for different trading styles and strategies. Traders can use Demark's lines to identify potential turning points, set stop-loss and profit targets, confirm other technical indicators, and combine them with different trading strategies.