In order to make use of tax-loss harvesting, the investor cannot violate the IRS' wash sale rule. That is, the investor cannot sell an asset at a loss and buy a "substantially identical" asset within the 30-day period before or after that sale. Doing so will invalidate the tax loss write-off. A "substantially identical security" is defined as a security issued by the same company (e.g. its class A shares vs. its class B shares or a convertible bond issued by the company), or a derivative contract issued on the same security.