defensive techniques. The rule of eleven
I have briefly described a few defensive techniques here although there are more. One useful defence against a no trump contract is known as the rule of eleven. It usually wise to lead a card from your longest suit against a no trump contract because if you and your partner can run the declarer out of that suit, all your "long", small cards will be masters if you get the lead again. The question is, which card to choose and that's where the rule of eleven comes in.
If you start with the two and work upwards, the ace becomes the "fourteenth" card in the suit as shown on the diagram above. Assuming you have the opening lead, the correct card to lead is the fourth down from the top of the suit. For example if you hold the cards shown here you would lead the ten of diamonds: (fourth down from the top of the cards you hold)
Your partner looks at the dummy hand and notes how many cards, higher than the one you led, are in that hand. They then subtract that number from eleven. The answer tells them how many cards the declarer AND themselves hold which are higher in value that the one you led. They can see how many they hold so they subtract this from the number they just arrived at and that's how many high cards in the suit the declarer has. In this example, the only card higher than the one you led is the ace so declarer must have this. It only represents one stop in the suit so next time either of you get the lead you will lead a diamond and make five tricks!
As a simple example suppose your partner leads the seven of a suit. Dummy is exposed and you see the ten and nine. First subtract seven from eleven = four. There are two higher cards in dummy and you hold the king. That accounts for three so declarer can only have one card higher than the seven.
Try it with a pack of cards. Deal them out as in the game and pretend you are leading. Pick your longest suit and lead the fourth down from the top of your holding. Now pretend you are your partner and use the rule of eleven to see how many higher cards the declarer has without looking at that hand.
A couple of other points. It probably won't do you much good to make this lead if you can't get the lead again because you have a very poor hand. Second point. Declarer can do the same calculation to work out how many high cards your partner has although it won't help them very much.