@ledzskie The engagement ring is the one the guy (or girl, gender equality ftw) give their love interest when they propose. That's the ring you see in them's romantic scenes where the guy gets on one knee and opens the small box to reveal the ring. It doesn't come in pair. Wedding/Marriage rings are usually exchanged at the wedding and come in pairs.
Incidentally, this also confused me as a kid, and I ended up thinking that after those romantic proposals you see in movies, the guy just yoinks the ring off the girl's finger to give it back at the wedding xD