The Arms Index (TRIN) is a market breadth oscillator that was developed by Richard Arms. It is also known as the Trading Index, hence the abbreviation TRIN. It relates advancing and declining stocks to their respective volume flows by dividing the
Advance/Decline Ratio by the volume ratio. When it rises the market is said to be weak and vice versa. The value 1 is key and crossing it generates a signal. A value above 1 indicates declining stocks have the upperhand in the volume flow and vice versa. The indicator can be used to spot overbought and oversold situations and is best used in combination with other analysis techniques.