Tournament sideboard rules

[RevanJJ]

I am curious, is it that you can sideboard only between games in a single round and not between rounds? 
That's what someone at the LGS I go to said and I wanted to make sure that that is correct.
Thanks

[Gorzo]

That is correct. When you face a new opponent, you must have your main deck ready for them, without your sideboard cards in it. After game 1 of your match with that opponent, you may start to sideboard in and out some of your 15 cards from your sideboard to help you.

RESOLVED

[RevanJJ]

Cool. Thx bro.

[Imdowd80]

Why is that? Wait hold....is it because you may have a buddy that has played your opponent so you can adjust accordingly, where as your opponent was not granted similar info.   

[Gorzo]

Pretty much. We're talking tournaments, so we're talking serious business. It would be easy to get info from friends or from your own eyes about your opponent before you sit down, and give yourself an unfair edge with your sideboard before the first game. This rule is in place to eliminate this possibility of a form of cheating.