A question has arisen from a local discussion about note taking and in particular the section in that talks about you have to show that your note sheet must be empty at the start and visible through out the match. The situation tha is causing discussion is, can a player make notes on the underside and keep that hidden?

From the MTR:

. Players do not have to explain or reveal notes to other players

There's not an annotated MTR (yet). My take on the philosophy of this is - Players are allowed to take notes, but these notes can be considered private information (not explained nor revealed). However, the sheet itself should be visible, so the opponent can verify that the sheet started blank, and the player is not consulting any notes from outside the match.

I'd consider it acceptable to keep notes on one side of a sheet to keep them hidden from the opponent, as long as the opponent could verify that both sides of the sheet were blank before the match begun.