We play a very extended version of Stayman. When our partner opens 1NT, most of he time that we hold both majors (4-4, 5-4, 5-5, 6-4, 6-5), we'll start with 2C. Here's our response structure after starting 1NT - 2C:
2D = no major
2H = weak hand, both majors; pass with better hearts, bid 2S with better spades
2S = invitational with 5 or 6 spades (may or may not have four hearts)
2NT = invitational (should have a four card major)
3C, 3D = natural, game forcing
3H = five spades, four hearts, game forcing (Smolen)
3S = five hearts, four spades, game forcing (Smolen)
3NT = to play
4C = 5-5 or better in the majors; opener bids his better one
4D = four spades, six hearts (Delayed Texas Transfer)
4H = six spades, four hearts (Delayed Texas Transfer)
4NT = invitational to 6NT
2H = four hearts, will only have four spades if a maximum hand
2S = invitational with 5 or 6 spades (tends to deny four hearts)
2NT = invitational with four spades
3C, 3D = natural, game forcing
3H = natural, invitational
3S = artificial slam try in hearts with no minor suit shortness
3NT = game values, four spades; opener can correct to 4S with 4-4 in the majors
4C = splinter raise of hearts
4D = splinter raise of hearts
4H = to play
4NT = invitational to 6NT (to ask for keycards in hearts, you need to start with 3S)
2S = four spades, can have four hearts if non-maximum
2NT = invitational with four hearts
3C, 3D = natural, game forcing
3H = artificial slam try in spades with no shortness
3S = natural, invitational
3N = game values, four hearts; opener can correct to 4H with 4-4 in the majors
4C = splinter raise of spades
4D = splinter raise of spades
4H = splinter raise of spades
4S = to play
4NT = invitational to 6NT (to ask for keycards in spades, you need to start with 3H)
This is simpler than it looks if you just remember that three of the other major is an artificial raise of opener's major. Another good system is what I play with my good friend Drew:
1NT - 2C - 2M -
3 Other Major = artificial slam try in partner's major with a side singleton
Next step = asks for singleton
4C = quantitative slam try with a fit
4D = keycard ask in partner's major
4NT = quantitative slam try without a fit
As with all other agreements---- a bad agreement is better than none at all! Do you know what your partner would have for these sequences? Talk it over!