In the United States, many American citizens, permanent residents (i.e. "green card" holders), and some eligible non-immigrant workers hold a Social Security Number, or SSN, to track wages and calculate Social Security Benefits of various types (e.g. Retirement, Survivor, and Disability Benefits). The number also serves as a means of helping identify and pay an individual, especially via the last 4 digits. My friends and I worked with them in the past, and wanted to share some considerations, and even a couple gotchas, we've seen along the way.