Wash sale

A wash sale occurs when a security is sold at a loss and a substantially identical security is purchased within thirty days before or after the sale, disallowing the loss for tax purposes.

The rule applies across all of a taxpayer's accounts, including a spouse's and including retirement accounts — which is why harvesting decisions have to be made with the household in view rather than one account at a time.

This is one of the strongest practical arguments for holding a client's relationship on a single system: a wash sale created by an account the advisor cannot see is still a wash sale.