HC/speculation: Meneldur was the first King's Heir to marry within the line of Elros.
So, a quick reminder: Tar-Meneldur is the 5th king of Numenor, and his son is Tar-Aldarion who famously did not marry within the line of Elros, which caused a lot of trouble, and he later made marrying-within-the-line a legal requirement for further kings.
Why do I think Meneldur did marry within the line? First, with how Aldarion and the narrative speak about it, marrying like that seems to be a thing that happened before. Also, Aldarion's mother is present in his story for quite long and doesn't seem to get old significantly earlier than her husband. And the line of Elros lived significantly longer than average Numenorians (about 400 compared to about 200).
Why do I think Meneldur was the first one? It's clearly either him or Tar-Elendil (his father), because before it would mean first-cousins, which was not done. And even for Tar-Elendil, "line of Elros" means marrying his second cousin, which is legal but probably seen as weird. Also, assuming that everyone had 4 children (which seems a good estimate), he would have 12 second cousins, so about 6 of them female, which is not a big pool to chose from.
Meneldur, on the other hand, would have about 12 ladies to chose from, and third-cousins are much more normal kind of people to marry.
Also, more importantly, Meneldur's position as King's Heir was... odd. He was the first one to not be the eldest child. He had two elder sisters, and so there must have been a lot of discussions whether the eldest child should inherit (the descendants of the people of Haleth would prefer it, I think) or the eldest son (the two other groups would prefer this probably). Marrying a descendant of Elros could also have been a way to strengthen Meneldur's position.
Numenor was peaceful, so it's not like there would be a rebellion, but peoiple would complain. (They were right. I love Meneldur but Silmarien should have been the Queen. Later history proves it.)
*checks Tolkien Gateway*
Tar-Meneldur's wife, Almarian had her first son in SA 700 and lived at least until SA900, so yes, definitely sounds like the line of Elros.
Oh wait, this would mean that Veantur (or his wife) was from the line of Elros. Which... why not. It makes sense.













