Lately I've had some timely questions about a practice that seems to have become common especially among novice and intermediate players. I had an email from a director in Palm Desert who has noticed this practice, and a question about it at my most recent summer bridge class.
Players, as dummy, have been routinely reminding declarer which hand they're in. The common thing is tapping the table to remind declarer that she's "on the board."
When it was brought up in class, I said, "Well it has to stop!" which got a bit of a chuckle.
Let me go into detail about dummy's rights and privileges. They're quite explicitly called out in the Laws of Duplicate Bridge.
First, there's Law 42 B. "Qualified Rights" paragraph 2.:
2. He may try to prevent any irregularity.