Regardless of your bridge partner's opening lead, you want to be prepared when it comes your turn to play third hand, which means that you're the third person to play to the trick. You need to know what to do when your partner leads an honor card or from a short suit.
When your bridge partner leads an honor card
When your partner leads an honor card (the ace, king, queen, or jack), she expects a little help from you in the form of an attitude signal. An attitude signal is a play that tells your partner whether you like the suit she's led.
You can make two types of attitude signals to convey your feelings about your partner's lead (and neither deal with glaring across the table!):
An encouraging signal: By playing the highest spot card (any card that isn't an honor) you can afford, you indicate that you want your partner to continue the suit.
A discouraging signal: By playing your lowest spot card, you tell your partner that you have no interest in that suit.
What determines which signal you use? Broadly speaking, if you have an equal honor in the suit that your partner has led, you give an encouraging signal. If you have three or more worthless cards in the suit, you give a discouraging signal.
Sometimes the spot card that you play may not be all that easy for your partner to read; a 9 is a big card and a 2 a small one, but what about a 5 or a 6? Sometimes your partner has to wait to see the second card you play in the suit:
If the second card is lower than the first (a high-low signal), it's considered encouraging and asks your partner to continue the suit.
If the second card is higher than the first (a low-high signal), it's considered discouraging.
When your bridge partner leads a short suit
You have to be a bit of a detective to spot a short suit lead. When your partner leads something like the 5, you may have to wait until you see your partner's second card:
If his second card is a 3, a high-low: A good chance exists that your partner has led high from a doubleton, so you try your darnedest to make your partner happy by leading the suit a third time so he can trump. If your partner has bid the suit and then leads high-low, your partner can't be showing a doubleton; he's showing you a five-card suit. As third hand, you're expected to play third hand high when your partner leads low.
If your partner leads a 5 followed by a 6, a low-high: Your partner can't have a doubleton; your partner is leading low from an honor. Keep that fact in your memory bank in case you want to return the suit.