is it legal to use a seperate board?

Not really, because OTB you can't have a separate board to try out different lines as an aid to visualisation.

Nobody is talking about additional lines, just replicating.

Merely replicating moves might technically be against the rules, but that would be unenforceable and pretty unimportant.  Using a physical board as an analysis board for trying out different lines is clearly against the rules.

Agreed. Some people literally like to set up the board to practice looking at a 3D board. I just think saying it’s against the rules is nitpicking.

I was thinking the exact same thing. Which is why MGleasons response makes so little sense to me. What better way to replicate over the board, than over the board? I had to use a real board to duplicate the moves when I first started playing on chess.com because the computer screen was so difficult for me to get used to. I was so used to playing on a  real board, I would make very embarrassing moves, simply because I just wasnt used to "seeing" a chessboard that way. 

If anything I would think chess.com would encourage using a real board for people who cant get the hang of computer screens because losing easily to someone rated hundreds of points lower could be considered rating manipulation.  I think the same things applies to the different colored boards, and different style pieces. If someone (like me anyway) were to play a blue and white board with odd looking different colored pieces, I wouldn't be able to "see" the board and pieces like I'm used to. So I would start losing badly to people rated far below me. If it's something that people are familiar with, that helps them play their normal game, I cant imagine any reasonable reason to be against it. 

@IfPatriotGames, see my most recent comment.  If you're using a physical board as an analysis board to try out different moves as an aid to visualisation, that's definitely against the rules (albeit difficult to enforce).

If you're just copying the moves from the game to see the position on a physical board rather than on-screen, it might technically be against the rules but isn't a big deal.

I saw your most recent comment. I also saw your comment about live chess being meant to replicate otb chess as closely as possible. I also saw Scotts comment about a separate board imitating otb better than a computer screen which is obviously true. What doesn't make sense to me, at all, is how using a separate board to duplicate moves could technically be against any rule. What rule is that technically against? How can someone get closer to over the board chess than over the board chess? 

As I said before, I had to use a separate board when I started playing online games because chess on a computer screen was so foreign to me. I was making moves that could only be explained as intentionally trying to lose, which I'm sure is also against the rules. There are probably other things chess.com could do to replicate over the board as much as possible, like forcing people to use the same colored board and pieces. In an over the board game, both sides dont use different style chess pieces, yet online it's allowed to help with visualization. Also, some people have bad eyesight, so they use glasses to help them see the board. That's an outside aide helping them that their opponent is not using. It just seems to me using a separate board is a non issue. Saying it's "technically" against the rules seems so unnecessary.

as MGleason says 'it might technically be against the rules but isn't a big deal.'

But when you have a discussion about the rules, you can expect nit-picking. Rules develop organically, and they are not always entirely logical.

But if a poster asks a specific question about the rules, expect unnecessary technicalities to be opened up!

So, if you find you need to make the moves on a physical board to help you visualise, fair enough, no is going to moan. But bear in mind that if you so much as touch a piece on your board, then you have to move it. And if you lift a piece and place it on a square, then that is the move you have to make on line. No changing your mind. Otherwise you are cheating !

(And no, you can't say j'adoube over and over again while you play out a variation !!)

I just build my own chess board and i play 3 day move game here , and let me tell you that it help improve and find those special move . But i think its fairplay since it take time to place the piece evertime time , i play mostly 20 game silmultaneously and i use the board for 2 or 3 of them , usually vs the best player .

Well i cant use the 2 board at all , i cannot see a thing on this flat screen lol , so i play with real 3d pieces set off chess.com on the screen at 45 degree angle , that is horrible , when i draw arrows it more then often turn the board in any directions and i have to replace it  , so my angle is never the very same ,  i am way better otb then on the screen . But since you are permitted to draw infinite arrow , would you consider permitting to have physical paper arrow to put on the board ? and those physical arrow could be potentially the same as look at what the board will look like after all the moves ? Because i read the rule and did not find any about trying some moves on the physical board being illegal while playing 3 days move game , is that correct ?

Outside assistance has nothing to do with assisting yourself.  Set up a board at the start of a game and use it to visualize.  There is no rule written or understood against it.

You're mostly playing daily.  In daily, you can set up a physical board and do your analysis on that board if you want.  You can also look at static resources such as books and databases.  If you're playing a particular opening, you could read a book on that opening.  This is all permitted in daily, and has historically always been permitted in correspondence chess.

Chess.com's daily chess rules are largely intended to recreate the history and culture of correspondence chess from the pre-engine era.  Books, collections of games, hours spent at an analysis board - those were all part of the culture of correspondence chess and were an accepted part of the rules.

Live chess is intended to mimic OTB chess, and so it has rules that are more in line with those you would see in an OTB tournament: no assistance of any kind.

No outside assistance? Mimicing online play with a otb setup is not outside assistance in my opinion.

Maybe legal if the player only uses it to measure advantages and disadvantages, and calculation is also fine.

You are struggling to make a case that does not exist.  First of all there is no way to determine, or control, someone who wants to do what you're suggesting in an online game.  In an OTB tournament it may be against rules to stand up or eat a sandwich at the board.  That can't be enforced online. The entire rule in C.C has to do with "outside help of any kind".   Someone playing on a cell phone might have a distinct visual disadvantage to someone on a large screen PC.  Visualizing with a board and pieces is in no way accepting outside help nor does it lend to cheating that couldn't simply be done otherwise.

Let me anwser you easely , what if someone who played chess all of his life otb , when internet was not invented and in 2019 (like me and im only 42) try it for the first time and got completly messup with the screen setup , losing my repair on the board , resulting in me not being able to use my full , normal potentiel for online game vs opponent wich are used to play on screen and online , i call it unfair . Chess was invented to play on a board , not on computer screen in the first place .

The rules are here: https://support.chess.com/article/648-what-do-i-need-to-know-about-fair-play-on-chess-com

They do not specifically address the case of someone using a physical board exclusively for the purpose of copying the position online, without doing any analysis on that physical board.

That's all true. When I first played online chess I just couldn't get used to the screen, so I would make moves that a 400 rated player would make. It just didn't work. Now I only use a real board for longer games. You are right, it would just take too long for short time control games. 

I know I am replying to the wrong comment, but I was gonna reply to you so:

using a physical board and playing out possible lines on it is obviously not allowed, and while it is pretty much impossible to enforce, it is known that it is breaking rules of chess and that it is cutting corners (and slows down progress a lot because it makes one’s visualization skills, which are crucial to chess, absolutely terrible).

I think the way of using a physical board that you meant was just leaving the exact position on the board to see it 3D and not look at lines. This doesn’t seem morally wrong, isn’t a shortcut, and probably shouldn’t hurt the game. I’d guess that the reason chess.com discourages that is because it is close to using lines, and it might encourage one who uses the 3D to look at lines on the board.

Imo using a physical board to play out moves is obviously bad for the game and both players (including the one looking at the moves themself), and the “no using physical boards at all” is just trying to safeguard against people playing out moves. But just replicating the board onto a physical one doesn’t seem… bad…

just my teenager uneducated person take

I've suspected a few players of setting up a physical board during longer games in order to shuffle pieces to look at variations. So they still don't play like an engine, but there's this long dramatic pause after they played terribly, then suddenly play like two-steps ahead for the rest of the game. It's an unfortunate unenforceable rule, because it's VERY hard to win a complaint about that kind of unfair play. The only thing you can do is play faster time controls such that they lose too much time setting up a variation board.