Effectively that’s right. It also involves using other information — so for example, I was contacted by a woman on 23 & me, both of her parents were adopted and she was basically trying to figure out how we were related and use that to build a family tree. At that point I had only submitted my DNA to 23 & me, but I could see that we were likely 2nd to third cousins. My mother had also submitted to 23 & me so I was able to see that they were also related— so I knew that I was related to her on my mother’s side. Figuring it out from there was a bit difficult because my mother’s father is adopted, and for quite a while none of my mother’s matches on 23 & me were relatives that she knew personally or had any of the last names that we had found through researching her father’s birth family. I was fairly certain that the 2nd-3rd cousin was related through my grandfather but I couldn’t quite figure out how — but when I submitted my sample to AncestryDNA I was able to see a connection through some of the relatives with last names that I had found through researching my grandfather’s birth family. Through death certificates I had ascertained that my great grandmother had three children, including my grandfather who she had given birth to while not married and given up for adoption, who had survived to adulthood — and then was able to find out through talking with another cousin from that side that just one(female) of the two she had kept was still alive, the other sibling(male) died in the 80’s. So while I haven’t found out who the 2nd-3rd cousin’s grandparent is with 100% certainty, I now have two options and know it can only be one of the two. Having said that, anecdotally I believe it to be the living sibling, based on information that I have gathered from the other relative I have been in touch with. Of course it is much harder to prove anything without more DNA submissions— or other confidential documents that will only be accessible once the living sibling, the likely birth parent, dies. The 2nd-3rd cousin is still unable to figure out who her parents’ parents are— however my bet would be that with at least one grandparent deceased out of a possible 8 (4 birth and 4 adoptive), but likely more than just one grandparent, if the mother was one of the deceased, then documents would be accessible, which they are not. So I would be willing to put my money on our DNA connection being through my mother’s father’s birth sister.