Ace, knave, ten, &c.
In trumps, play ten, or with cards in sequence with the ten, the lowest of the sequence.
In plain suits, play a small one.
The reason for the difference is that, in trumps a small card may be led from king, queen, &c.; but in plain suits, not. Hence as, in plain suits, the king or queen must be in the third or fourth hand, your strength would be wasted by covering.
Ace and small ones.
Play a small one.
As before observed, the original lead of a small card from strength is assumed.
If, after several tricks have been played, you particularly want the lead, or you suspect the possibility of a lead from a single card, or one trick is of importance, you would often be right to play the ace. Again, no rule can be laid down.
King, queen, knave, &c.
Play the lowest of the king, queen, knave sequence.