Ace, king, and small (in plain suits).
Lead king, then ace.
King is led before ace to inform your partner that you hold the winning card of your suit, in case you should think fit to change the suit, or in case the fourth player should have none of it. If your partner has none, he should not trump, for, even if you do not hold the ace, you want the adversary to play it, that he may not retain the winning card of your suit.
When opening a plain suit, headed by ace, king, after having been forced to trump, lead the ace first. If you begin with the king, and your partner happens to have none of the suit, he might trump the king, in order to lead again the suit you have already trumped.
If intermediate cards fall and you remain with the command and the next best, you inform your partner of the fact by continuing with the next best. Thus:—You lead king from ace, king, knave, etc. To the king, your partner drops the queen. You should next lead the knave.
Ace, queen, knave, ten, with or without others.
Lead ace, then ten, even though one of your others is the nine.
With more than four in suit, lead knave after ten.
With only four in suit, lead queen after ten.