Opener rebids the opening major

After 1H > 1NT > 2H or 1S > 1NT > 2S, there is no point for responder to bid the other major -- in the former case responder denied 4 spades; in the latter case if responder has 4+ hearts with good values he should have bid 2C/2D/2H (in case of 13+ points) or 3C/3D/3H (in case of 9-12 points). Hence responder bidding the other major would constitute an unusual suit to show 10-12 points with 3+ trumps (a hand which responder was prepared to bid 2M had opener rebid 2m after1NT), as opposed to a 3H rebid which is a general invitation with exactly 2 trumps. This is summarised below:

Response	After 1H > 1NT > 2H	After 1S > 1NT > 2S
2S	10-12, 3+ hearts (!!)
3H	Exactly 2 hearts, invite to 4H	10-12, 3+ spades (!!)
3S		Exactly 2 spades, invite to 4S

Response	After 1H > 1NT > 3H	After 1S > 1NT > 3S
3S	10-12, 3+ hearts (!!)
4H	Exactly 2 hearts	10-12, 3+ spades (!!)
4S		Exactly 2 spades