When, after your Opening bid, your Partner responds with a bid of a new major suit he promises 4 cards in that suit. However he could hold 5 or more cards in that suit.
If your RH Opponent then makes an Overcall or Take-out Double, you (as Opener) then have the opportunity to show 4-card or 3-card or less support using a Support Double (or Support Redouble).

A. With 4+card support you raise Partner's suit as normal : 1♣ - (Pass) - 1♥ - (1♠) - 2♥ = 13-15 1♣ - (Pass) - 1♥ - (1♠) - 3♥ = 16-18 1♣ - (Pass) - 1♥ - (1♠) - 4♥ = 19-21	W N E S 1♣ - 1♥ 1♠ 2♥
B. With 3-card support make a Support DBL, provided that (with 4-card support) you could have made a single raise of Partner's suit at the 2-level instead.	W N E S 1♣ - 1♥ 1♠ X
C. With 2-card support or less : Bid a new suit (even a "false suit" if necessary) Raise your own suit (showing 1 more card in that suit) Bid NT when balanced, with Enemy suit stopper. Pass, when weak with no option, Partner can bid again	W N E S 1♦ - 1♥ 1♠ 2♣

1.	W N E S 1♥ - 1♠ 2♣ X	West (Opener) ♠ K 8 7     ♥ A Q J 4 2     ♦ 8 5     ♣ A 6 3 Showing 3-card ♠ support
2.	W N E S 1♦ - 1♥ 1♠ 1NT	West ♠ K 10 9     ♥ 7 2     ♦ A K 8 5 4    ♣ A 6 3 Balanced, Enemy suit stopper, 2-card ♥ support
3.	W N E S 1♣ - 1♥ 1♠ 2♣	West ♠ J 10     ♥ 4 2     ♦ K Q 9 5     ♣ A K 9 8 3 13-15 pts : too weak to bid 2♦
4.	W N E S 1♣ - 1♥ 1♠ 2♦	West ♠ 10 8     ♥ A 4     ♦ K Q J 8     ♣ A K J 6 2 2♦ is a "Reverse bid", showing 16+ points. (Opener's 2nd suit at the 2-level is higher ranking than his Opening bid.)
5.	W N E S 1♥ - 1♠ 2♦ X	West ♠ J 7 5     ♥ A Q J 9 3     ♦ K Q 4     ♣ 6 2 Showing 3-card ♠ support
6.	W N E S 1♥ - 1♠ 2♣ 2♦	West ♠ 5     ♥ A Q J 9 3     ♦ K J 8 4     ♣ K J 9 When unbalanced Opener should not bid NT the 1st or 2nd round. Responder must bid again, so bid NT on your next turn.
© 2016 Michael Furstner