Long story:
I was born with three names - as an example, Jane Mary Doe. I never used the name Jane and was always called Mary so when I got married, I became Mary Doe Smith. When I got divorced I wanted to go back to my maiden name but leave out the first name since I never used it. The attorney drew up the paperwork and I went from being Mary D. Smith to being Mary D. Doe so the D stands for nothing
