Penalty for violating FIDE rule 8.1.2 - writing moves in advance At one of the chess league fixtures recently, one of the players was writing moves in advance of moving the pieces on the board.

As per the FIDE rule 8.1.2 it's forbidden to do so, but in practice what's the penalty for the player who does it? Should it be a verbal warning and loss of the game on a subsequent attempt or something else?
Should it be a verbal warning and loss of the game on a subsequent
  attempt or something else?


It depends entirely on the circumstances, although a player would never be defaulted for the specific reason of writing the move before playing it. (I will return to this point at the end)

A big part of the problem is that years ago young players were taught to write the move down, do a final blunder check and then make the move. If the blunder check revealed a problem or they just had second thoughts they changed what was written and made the new move. This is the nub of "making notes".

First, if as an arbiter, I'm observing a game and the player is consistently writing the move, pausing for reflection then making the move then I will step in and warn him.

If the player is just writing the move and then immediately playing the move then I will not intervene unless the opponent complains but I will speak to player after the game and tell him that what he is doing is against the rules and tell him not to do it again. This is a general principle for very minor infractions when the arbiter intervening would disturb the players unwarrantedly.

Coming back to what I said at the beginning, that a player would never be defaulted for this, we can look at what happened in the Wesley So case which is analagous. He wasn't writing moves down but writing advice to himself about how to play. He was defaulted after 3 warnings but he was not defaulted for writing notes. 

He was defaulted for ignoring the instructions of the arbiter which is something different and, in the extreme case that a player continued writing moves down before making them after several warnings from the arbiter, this would also happen. The player would be defaulted for ignoring the instructions of the arbiter. It may look as if it is the same thing but it is not.