Bridge & other card games/leading single trump card
I have heard that you can lead out a trump card if you have 2 or more of them, but you should NEVER lead a single trump card. Is this true and why should you not lead a single trump card?
Although I would not use the word "Never" it is usually not a good idea to lead a singleton trump. The reason is that it may make it easy for declarer to play the trump suit correctly when he might play it incorrectly if left to his own devices. For example, assume declarer has nine total trumps between his hand and dummy...AJxxx in his hand and Kxxx in dummy. You have a singleton and partner has Qxx. All other things being equal, the declarer, with nine trumps, will usually play to drop the queen by casings the ace and king. While this will frequently work, it will not work on this particular hand and partner will eventually get a trick with the queen. Declarer has no reason to take the finesse especially if the opponents have not bid anything to reveal their hands. Leading your singleton trump, however, makes the situation obvious for declarer and he will pick up the suit without any losers.
If you have a doubleton trump, then either your partner only has a doubleton so declarer will not go wrong anyway or, if partner has Qxx, then declarer now only has eight cards in the trump suit so the finesse is called for anyway and declarer should get it right on his own.
Other trump holdings by partner that might be compromised by a singleton lead are Qxxx, QJxx, Jxxx and others.
There are rare occasions, though, when leading a single trump might be correct. For example, you know partner has a long trump stack as passing your takeout double (1S-X-P-P-P), or opponents made a high-level sacrifice with few honors - planning to cross-ruff.