See also bridgebum.com and Barbara Seagram's article

Jordan 2NT (aka Truscott 2NT) shows an artificial limit raise or better by responder. It is used when partner opens 1-major and the opponents make a takeout double:

In this auction, 2NT shows 3 spades and 10+ points. This frees up the jump raise of 3♠ to be a weak, preemptive raise (4 spades, 5-9 points).
With balanced shape, 10-12 points and not fit, responder simply redoubles over the opponent's double.

Opener's Rebids

Opener's second bid is natural, barring any other agreed conventions. Note that opener's rebids are the same if the opponents pass or bid over 2NT:

• 3♣ Game try (3-4 clubs) asking responder to bid the game with strong clubs
• 3♦ Game try (3-4 diamonds) asking responder to bid the game with strong diamonds
• 3♥ If opener originally bid 1♥, then this is a signoff. If opener originally bid 1♠, then this is natural, showing 13+ points (game-invitational values).
• 3♠ If opener originally bid 1♠, then this is a signoff. If opener originally bid 1♥, then this is natural, showing a good 14+ points and 4+ spades. Since opener has bypassed 3♥, this is forcing to game.

Responder's Rebids

If opener has signed off in 3♥/♠, then responder can pass with a minimum.
If opener has bid a new suit, responder can sign off in 3♥/♠ with 10-12 points. With 12+ points but no slam interest, responder can sign off in 4♥/♠.

Example

Example

Example