"I heard you have a new girlfriend," I said to Cy the Cynic.
"I met this one at a chess club," Cy nodded.
"Don't tell me," I sighed. "She made the first move."
The opening lead -- the first move in the play -- can affect how declarer operates. Suppose West leads the jack of hearts against today's four spades. Take the ace and lead the ace and a low club. Win the heart return in dummy, ruff a club, draw trumps and ruff the last club. Then exit with a heart.
If East wins, he must lead a diamond to avoid conceding a ruff-sluff, and you can hope to guess right. (As the cards lie, you go down, having done your best.)
Now say West leads the jack of clubs, so the defenders do not threaten to set up a fast heart trick. Take the ace, lead a trump to your hand and return a low diamond toward dummy's jack.
When West has the queen, you can set up a diamond winner to discard a heart from dummy. If East had the queen, you would lead a diamond toward your king later.
You hold: S 6 3 H Q 8 4 D 9 8 7 6 2 C K Q 7. Your partner opens one club, you respond one diamond and he bids one heart. What do you say?
Answer: No action is attractive, but you should not pass one heart when your partner might have as many as 18 points and you have three honors in his suits. Game is still possible. Bid two clubs. Your partner may have opened with a "prepared bid" on a three-card suit, but then he will have to extricate himself as best he can.