A trial bid is used when the bidding goes for example 1♠ (p) 2♠ (p) and you want to invite game. Typically the simple major raise covers a spread of 3 or 4 hcp, and opener is unsure about game.
This is done only with the majors, as usually a new suit after minors shows stops for NT purposes. Bidding 3 of the major is the simplest trial bid, and this is fine if responder is minimum or maximum, but what does he do if he is in between? This is where most people make a trial bid in a suit. With a minimum, responder signs off in three, with a maximum he bids game, and in the middle makes a judgement as to whether his holding in that trial suit in an asset or otherwise.
Some people play "long suit with values" trial bids, eg KJx. Others play "long suit with nothing" trial bids, eg xxx. Yet others play "shortage" trial bids, such as x.
This information is a godsend to the opponents who are wondering what to lead! "Values" trial? - Can't lead that suit, it will either give tricks away or give a free finesse where the timing may not be there for declarer to do it himself. "Nothing" trial bids? - a clear-cut lead. You are safely leading through dummy to whatever partner holds.
Why help opponents? A third of the time responder is going to sign off in 3 regardless of your trial suit, a third of the time he will bid game regardless of your suit, and only on one third of the time will it help reach the right contract. But at what great expense! What you really should do is not show anything at all two thirds of the time, and only when it makes a difference show something that will NOT significantly help defence. Tricky.
The answer is to have a new trial bid.
This trial bid is the next step in the bidding : 1♠ 2♠ 2NT, or 1♥ 2♥ 2♠. The bid means nothing other than that it invites game. Responder bids game if maximum, signs off in 3 if minimum, and in between he bids a "no values" suit. Something like xx or xxx is ideal. This is the least helpful information to the defence's lead, and it assists opener to make the decision whether to bid game.
A minimum responder signs off in trumps. A maxiumum responder bids game. An in-between responder bids ...
After 1♠ 2♠ 2NT
3♣ is a nothing club suit
3♦ is a nothing diamond suit
3♥ is a nothing heart suit.
After 1♥ 2♥ 2♠
2NT is a nothing spade suit
3♣ is a nothing club suit
3♦ is a nothing diamond suit.
The reason 2♠ has to be the trial bid with hearts as trumps is to allow room to show a nothing spade suit.
However, this DOES NOT STOP opener using any other non-jump bid as an old-fashioned trial bid, provided there is something specific he wants to know. By partnership agreement, this could be 3 cards with one of the top 3 honours, or a no-values shortage, but it needs to be an exact question that is asked. If opener bids the next step over trump agreement, this is the general "don't give away any information" trial, while any other bid asks that question about a specific suit. After spade trump agreement, any suit is the specific suit ask. After heart agreement, 2NT would substitute for the ask in spades.
Why not try a more modern trial bidding method, and stop making life easy for the defender on lead?
In summary, after 1M 2M, 2(M+1) is the game invitation bid.
Ray Green (fromagegb at gmail.com) revised Feb 2016