1H - 2H or 1S - 2S shows 6 to 9 point
1H - 3H or 1S - 3S shows 10 or 12 points
1H - 4H or 1S - 4S shows 0 to 9 points and a 5-card support.

It might seem logical to use a raise to 4H to show 13 or more points. In fact, some partnerships do use this approach. There are a couple of reasons why it isn’t the recommended strategy. First, opener could have a strong hand and be interested in slam, if responder has an opening bid. If you jump to 4H, there’s not much room left for any investigation, if it’s a borderline decision. Secondly, most players like to reserve the jump to the game level as a preemptive raise, showing excellent trump support but a weak hand.

The Jacoby 2NT response to 1H or 1S establishes the trump suit and commits the partnership to at least the game level.
Since the partnership is already headed for game, the only issue is whether the partnership belongs at the slam level. To help responder evaluate whether the partnership can make a slam, opener makes a conventional response to the 2NT bid, describing both the strength and distribution of the hand. The key focus on distribution is whether or not opener has a short suit — specifically, a singleton or a void, or maybe another long suit (at least a 5-card suit). Knowing about the opener's distribution helps responder evaluate the prospects for slam. Responder can see whether the hands fit well together.

• Opener’s first obligation is to show a second suit.  It must be a good five card suit that contains at least one of the two top honors.  Opener would signify that by jumping to the 4 level in their second suit
• Opener’s second obligation is to show shortness.  With a singleton or a void in another suit, opener would bid three of the suit in which they have shortness (not an Ace or a King)
• Opener’s third obligation is to show the strength of their hand. If opener has no second suit and no shortness, they will show the strength of their hand.

4th level of a new suit shows 5+ card suit with at least A or a K in that suit. This is such a powerful bid, it shows 10 + cards in both of the suits (5 card major + another 5 card suit ) so they must have a shortness in the other 2 suits.

3rd level of a new suit shows singleton or void in bid suit.

 3 of agreed major = Maximum range opening (18+ points)
 3NT                        = Medium range opening (15-17 points)
 4 of agreed major = Minimum range opening (12-14 points)

Once you have made your Jacoby 2NT (game forcing raise), your job is NOT finished !! After the opening bidder has provided you with additional information, you now reevaluate your hand based on that information.

- To show a lack of interest in slam, responder to the opening bid goes directly to 4 of the agreed major
- To  show interest in slam, either bid 4NT  (Key Card Blackwood 1430) or cue bid your cheapest Ace or Void to show interest in bigger and better things. (You will learn about cuebids later)

Jacoby 2NT is off in competition. After interference over opener's 1H/S, a 2NT bid by responder is commonly used as a limit raise (the Jordan 2NT convention).