Also known as a “trigger line”, it is a moving average of another indicator that is used to generate simple buy and sell signals. Probably the most used signal line is the one that is built into the MACD Indicator display. The signal line is the exponential moving average of the MACD line. A buy signal is generated when the MACD line crosses above the signal line and a sell signal is generated when the MACD line crosses below the signal line.