The lead of an eight or a seven (except at no-trumps) is more apt to be from a short than from a long suit; combinations in which these cards would be fourth best are seldom opened by good players.
It will be easier for your partner to read your lead, if with long weak suits, headed by jack, ten, or nine, you lead the fourth best card. These cards can be led to indicate a short suit.
You can determine the length of your partner’s suit by noticing the fall of the low cards.
When it is evident that your partner has led the lowest card of his suit, his lead indicates exactly four cards. If your partner is leading from a long suit and plays down on the second round, he holds more than four cards.
EXAMPLES OF INFERENCE FROM OPENING LEADS
A is the leader, Y the dummy, Z the dealer.
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A’s lead of the ace denies the king and probably the queen; both of these cards should be placed in the dealer’s hand.
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Third hand should play ace on queen; king is in the dealer’s hand. A has probably led from queen jack ten.