Used only in the sequences:
2 is Forcing. Showing a minimum of 9+ HCP
1X - 1Y - 1NT - 2
or 1X-(1Y opp)-*(Partner)-1NT-2
or 1X-(* opp)-**(partner)-1NT-2
Opener's 1NT rebid shows 12-16 HCP
Opener's continuations are:
3Y Shows 3 card Y and 15-16 HCP
3Ma Shows an unbid 4
card major - in a 4432 distribution where 'Y' is 2 card
3 Shows 16 HCP
If not 'X' or 'Y' shows 4 Diamonds e.g. 1-1 - 1NT - 2 - 3
& 15-16 HCP.
Also 1-1H - 1NT - 2 - 3 Shows 5 diamonds &
2Y Shows 3 card support for 'Y' and 12-14 HCP
Shows an unbid 4 card Major suit - in a 4432 distribution
2X Shows 5 card suit & 12-14 HCP
'Ma' in the above sequences is not 'X' or 'Y' ('Y' may be the other Major - Ma') & 'X' is not Diamonds.
2NT Shows 15
HCP or a very good 14 but only as long as 2X & 2Y above are not valid
2 None of the above, 12-14 HCP
Now responder has a clear view of partner's strength and distribution,
the final contract can be selected. If responder is strong enough (2
is unlimited) he can use ERKCB.