Carding conventions like bidding conventions are public. Let me give you some examples. When defending a trump contract and you, as a defender, lead a side sui

Carding conventions like bidding conventions are public. Let me give you some examples. When defending a trump contract and you, as a defender, lead a side suit, what card you lead describes your holding. An example is you tend to lead top from an honor (AKQJ are honors) sequence. So if you have QJxx you'll tend to lead the Q. Doing so denies the K (as KQxx has a different lead). It may or may not deny the A depending on what you've agreed upon. So with AQJxx you might lead Q ow low depending on your convention. Likewise your partner's card tells you something. For one you need to know if they're carding count (how many of that suit they have) or attitude (whether they want you to continue that suit or shift). So upside down attitude means you respond with a low card if you like it (standard ir high as a positive signal but most consider that an inferior treatment as you may waste valuable high cards). The leader can infer the meaning of your signal based on their holding and what's in dummy. Like the leader might have 32 and partner plays the 4. Leader knows this is a positive signal. Declarer who cannot see your 32 might not know this. This is an example of imperfect information mentioned in the article. Declarer knows what conventions defenders are playing. Hidden understandings are illegal. You can lie to your partner but if, say, you discourage a lead and your partner continues it and that's the winning play then you may get penalized for that in competitive play.