How do you properly address a letter to a widowed woman?

When addressing a letter to a widowed woman, you can use "Mrs." followed by her late husband's last name, if she prefers that designation, or simply use her first and last name. For example, "Mrs. Jane Smith" or "Jane Smith." If you know she prefers not to use "Mrs." due to her status, you could address her as "Ms." followed by her name. Always consider her personal preference if you're aware of it.