Comment by sgerenser

Indeed, the wording is “substantially identical”, which is important. 2 different ETFs that track similar, but not identical indices (e.g. S&P500 vs Russell 1000 large cap, for example) are clearly not substantially identical, and make great tax-loss harvesting pairs. There’s tons of case law, opinions from tax experts, and automated tax-loss harvesting tools from a variety of brokers that agree with this viewpoint.