Odd - Even Discards
Signaling With the First Discard
There was a time long ago when odd/even discards were exotic and few players knew what they were, but most players are using them today.  Maybe something better will be developed someday, but for now they are the best.  Even so, many players today are still using either Standard Discards or Laventhal Discards.  In my estimation they are at a disadvantage.

Odd/Even Signals

The ACBL has a regulation that says you can only signal on your first discard.  That seems odd, but you seldom get the opportunity to use a suit-preference signal later in the hand, so it's not much loss.

This is the agreement:
  • At your first opportunity to make a discard, if you play an odd-number low card, such as the 3, 5, 7 or 9, you are asking your partner to please lead the same suit.  If you discard an odd club, such as the seven, you would prefer a club lead the next time your partner is on lead.

  • If you discard an even spot card, such as the 2, 4, 8 or 10, it will discourage youer partner from leading that suit, but more importantly, the size of the even card is a Suit Preference signal.  A small spot card, such as the 2 or the 4 says, "Partner. please do not lead this suit... Lead the Lower of the two remaining suits.  Perforce, a high and even signal, such as the 8 or the 10, asks for the higher of the two remaining suits.
Note that the six spot is not included in these signals because it is neither high nor low, and cannot be interpreted as a signal.  Probably the best you can understand about a discard of the six is that either partner does not have a card he can use as a signal, or he has no preference.

Laventhal Discards

Better than Standard discards, but not as good as Odd/Even discards are Laventhal signals.  If your opponents are using these signals, this is an explanation.
    If an opponent discard any spot card, odd or even, high or low, it tells his partner not to lead that particular suit.  Instead, the size of the discard sends a signal as to suit preference.  Again, the six shows no preference.