In technical analysis, it is a complex price pattern based on Fibonacci numbers/ratios. It is used to determine buy and sell signals by measuring price retracements of a stock's up and down movement in stock price.


Source: www.harmonictrader.com

The above Gartley example shows an uptrend XA with a price reversal at A. Using Fibonacci ratios, the retracement AB should be 61.8% of the price range A minus X, as shown by line XB. At B, the price reverses again. Ideally, retracement BC should be between 61.8% and 78.6% of the AB price range, not the length of the lines, and is shown along the line AC. At C, the price again reverses with retracement CD between 127% and 161.8% of the range BC and is shown along the line BD. Price D is the point to buy/sell (bullish/bearish Gartley pattern) as the price is about to increase/decrease.