Is the number of cards in a players sideboard derived or private information? I know that the number of objects in a zone is derived information, but the sideboard is outside the game so that doesn't seem to fit. Also I know that during the pregame procedure the player can count their opponents sideboard. What I don't know is if it's legal for a player to count their opponents sideboard during a game.

I thought a question like this was asked before, but I couldn't seem to find it. Any help would be appreciated.
Thanks
Cris Plyler

Originally posted by MTR 3.15:

Before each game begins, players must present their sideboard (if any) face down and, if requested, allow their opponents to count the number of cards in their sideboard. Players are not required to reveal how many cards they have swapped from their main deck to their sideboard.

What that doesn't tell us is what can happen during a game. I'll ping a few folks, see what the ‘O’fficial consensus comes up with.

Number of cards in a sideboard should be considered Derived Information; that will be more clearly stated in a future version of the MTR.