Jacoby 2NT ("Original")
Most tournament players use "Jacoby 2NT" in conjunction with their 5-card major system. After an opening of 1-MAJ, a jump to 2NT is conventional. It promises a GF hand and at least 4-card support.
This convention should not be used by a passed hand and should not be used if there is interference by the opponents. I can't warn you enough how important it is when employing any convention to cover this vital issue (on by PH?--on in comp?).
In the "original" (most popular, still) version of this convention, this is the schedule of rebids for the opener after the Jacoby 2NT reponse:
4 of the trump suit=minimum, no shortness
3 of the trump suit=non-minimum, no shortness
3 of a new suit = shortness (singleton or void)
3NT=varying ways to play this (usually shows 5-3-3-2 -- and the partnership needs to discuss the HCP range).
4 of a new suit = good side 5-card suit (some people instead use this to show a void)
At any point, by either player, bidding 4-of-the-major which was opened is always the weakest (sign-off) action. Other bids after the Jacoby 2NT and opener's rebid are control-bids.
As stated above, this convention is never on if they interfere after our 1MAJ opening. However, we do need to cope if they interfere after the 2NT response:
Opener's double = Shortness in suit doubled
Opener's new suits = natural or a control
3NT = Balanced Maximum (Ace or King in their suit)
Opener's Pass = Nothing special--flat hand, could be 5x3x2 awful after which, responder's X=penalty
Opener's jump to 4M = dead minimum, but 6x3x2