How do you formally address a widow?

A widow can be formally addressed as "Mrs." followed by her late husband's last name, for example, "Mrs. Smith." In cases where she prefers to use her own first name, you may address her as "Ms." followed by her last name, such as "Ms. Johnson." It’s always best to follow her preference if you are aware of it. If in doubt, using "Ms." is generally considered respectful and neutral.