All partnerships should have a discard system to indicate which suit they would like led. Opponents are entitled to ask the meaning of any discard, but you need only give that it is indication of interest in that suit. Explain what you have agreed, rather than stating which suit you are expected to lead.
The first discard is the significant pointer, because subsequent discards are often made with a view to protect another holding. Occasionally a second discard in the same suit can qualify your first discard. Make sure that both players in the partnership are playing the same system!
The most basic system is to play a high card in the suit you want led. There is no doubt about its meaning but can be wasteful. Take a holding in s of K105. Playing the 10 may cost you a trick in subsequent play. Just throwing away a card in a suit you do not want led is less wasteful, but can put partner on a guess as to which suit. However discarding a card in a suit that you do not want led is the basis of most discard systems for more experienced players. In all cases the suit that you are now void in is obviously discounted, and likewise trumps. Discarding when opponents are playing a no-trump contract can give you an extra option.
A run through of systems in use. All systems fail sometimes because you have “wrong “ cards for the system. Any system that allows two choices of card for the same suit, has a definite advantage over basic systems.
1. Revolving discards. Here the suits are considered in a “circle”. In a contract high indicates a lead of a diamond, and a low suggests a spade suit . The spade suit can also be indicated by a high card in s
2. McKenney. The discard indicates a suit by suit rank, a high card in the discarded suit suggests the higher ranked suit of the two remaining suits, and a low card the lower ranked suit . Suppose hearts are trumps, and you have to discard on a heart lead, then a low suggests a lead of a and a high suggests a lead of a .
Remember the suit that forces you to discard and the suit you actually discard do not count so the choice is always between the other two suits.
3. Odd even McKenney. This is a variation on McKenney. An odd card in the suit you discard is a request to lead that suit, and an even card is normal McKenney. Again hearts are trumps and you have to discard. The 3 suggests lead a , whereas the 2 suggests a and 8 suggests a . Of course you could ask for a lead by discarding a high even card in s.
4. Dodds. This is somewhat similar to odd even McKenney. An even card asks for that suit, and an odd card asks for the suit of the same colour. Again hearts are trumps, so 2 asks for a , and 7 asks for a (same colour suit)
5. Goldfoot. This a local system, where by a discard of a 2,3 or 4 suggests the next suit up, a discard of a 5,6, or 7 suggest a suit two removed, and the 8, or 9 suggest the suit below (think of it as suits in a circle again)
Suit preference signals in giving partner a ruff. The usual system is based on McKenney. You are leading partner a suit for her to ruff. A low card suggests the lower ranked suit should be returned and a high card the higher ranked suit. This is particularly suitable if you have entries to give partner more than one ruff. Again hearts are trumps, and partner leads a singleton , won by your ace. You hold A, so you lead the highest you can spare to ask partner to return a for you to win with the ace and lead s for a second ruff. A return of a low would suggest a return. If you hold no quick entry card try to indicate this by playing a neutral card in s or indicating dummys ace as your "entry"
Neutral suits. Often you have no suit you would like led, so to try to indicate this to partner, by suggesting a lead (by a discard) of a suit you obviously do not want partner to lead , by indicating a suit where you cannot win a trick.
You have to discard something, so try to keep a possible winning card in dummy’s longest side suit, even if that means not being able to indicate to partner which suit to lead. Your discard must not weaken a holding in that suit.
Discards cannot be an absolute indication of the suit you want led, partner sometimes has to work it out for herself.