When an uncontested auction begins 1m-1M-2m, opener is showing a 12-14 HCP hand with a 6-card or longer minor. At this point, responder's hand is unlimited. Responder may want to:
- Play in a partscore
- Invite game
- Force to game
- Make a slam try
- Look for the right stain
- Look for support for their major
- Assess whether notrump is feasible
- Attempt to right-side a notrump contract
The problem is there is not enough room for natural bids to show their shape and their strength.
The 1m-1M-1NT auction is analogous. If playing a 15-17 opening 1NT range, opener's 1NT rebid shows a balanced hand with 12-14 HCP. Opener is showing a minimum opener with a Responder needs ways to signoff, invite, and force. Many partnerships use a conventional New Minor Forcing bid to allow responder to show a wide variety of hands and gather the information they need to place the contract. 2-Way New Minor Forcing is easier and better. It uses 2 conventional bids.
When opener rebids their minor, showing a minimum opening hand with a 6-card or longer minor, responder has similar needs. See the auction below for an example.
Which hand is South showing?
- ♠Q84 ♥AK62 ♦Q87 ♣962
- ♠Q84 ♥AK62 ♦A87 ♣962
How would you bid the other hand? You don't want to guess whether a bid is invitational or forcing.
3rd Suit Forcing is an easy solution.
If responder has a game forcing hand and isn't sure which game is best, responder bids the cheapest unbid suit as an artificial game force. This makes responder's bid of the cheapest unbid suit alertable.
With that agreement, South would rebid 2♠, the cheapest unbid suit, as an artificial game holding hand 2 (13 HCP, ♠Q84 ♥AK62 ♦A87 ♣962),
That leaves room for opener to show a club stopper to help determine whether 3NT is viable.
The raise to 3♦ is clearly invitational because South didn't use the 3rd Suit Forcing artificial game force.
North can pass the invitation with a minimum. If North is closer to the maximum of their range, North can make a move towards game by bidding 3NT, showing 3-card heart support, or showing a spade stopper. Since opener's rebid of their 6-card or longer minor is typically a playable contract, responder shouldn't be eager to bid again unless they believe a game may be available.
Partnerships who already play 4th Suit Forcing to game will be comfortable playing 3rd Suit Forcing to game.