Defense basics: What card to lead

Let's standardize on some standard leads. This helps you win tricks as well as giving your partner clues as to what's in your hand.

Leads when defending against a no-trump contract

The top card of a three-card or longer sequence that includes an honor.
KQJ    109843    AKQ7

This includes broken sequences:
KQ1063    10975

and internal sequences:
AQJ10    Q109854

If your partner bid the suit, you can lead the top of a two-card sequence:
KQ4    109543    

From a long suit without a suitable honor sequence: the fourth card down.
A98532    KQ87

From a doubleton: the top card.
76    K4

From three low cards: the middle card, then up, then down (aka MUD).
765    932

From three cards headed by an honor: the low card.
K98    AJ3

Against a trump contract, some principles are different.

Lead the top card of a two-card or longer honor sequence.
KQ4    109543    AKQ7

From three cards headed by an honor: the low card, unless that means leading away from an ace.
K98    AJ3

From a long suit without a suitable honor sequence: the fourth card down, unless that means leading away from an ace.
A98532    KJ874