Jacoby Two Notrump
A 2NT response to a Major sit opening promises at least 4-card support and at least opening hand strength. It is absolutely forcing to game.

Opener's second bid describes her hand further:
  4 of the Major shows a minimum featureless hand
  3NT shows extra strength and a 5-card trump suit
  3 of the Major shows extra strength and a 6+ card trump suit
  3 of any other suit shows shortness (singleton or void)
  4 of any other suit shows a second 5-card suit

Splinter Bids
A double-jump-shift as a response to partner's opening 1 of a Major suit is a Splinter bid. It shows opening hand strength, at least four card support for the Major, and a singleton or void in the "splintered" suit. It is absolutely forcing to game.

BakerBridge has complete lessons for these conventions.

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