Opening one of a suit, 1NT overcall

East’s obvious opening bid is 1♥.

South has a natural 1NT overcall if playing 15 - 18.

If North passes East will probably rebid 3♥.

If North starts an invitational sequence with 2♣ then NS will probably reach 4♠.

Post-beginner and above

Opening at the four-level, double, North passes

Following two passes East might just try 4♥ in third seat.

If the double is primarily for penalties North will be torn between pass and 4♠.

What happens?

The frequencies quoted in the following are rounded from the 400 plays of the hand (or thereabouts) for which contract information is available.

Roughly 160 played in a spade contract and 220 in a heart contract, frequently doubled in both cases.

In theory 4♠ can be made.  In practice 55 pairs made ten tricks (scoring 91% to NS if bid and 64% if in a part-score), and 65 made 9 (60% in a making part-score), but these scores have to be set against 45% for one down and 19% for one down doubled.

4♥ can in theory be taken two down (66 pairs scoring 77% to NS), but in practice was usually one light (143 pairs scoring 47% to NS and 53% to EW)

55 of those in a spade contract made 10 tricks, 65 made 9 tricks and the rest made 8 or less.

7 of those in a heart contract made 10 tricks, 143 made 9 tricks, and the rest made 8 or less.

Opening at the four-level, double, North bids 4♠.

East tries a 4♥ opener for size and South doubles.

If the double is primarily for take-out then North will bid 4♠.

If the double is primarily for penalties North might still prefer to have a go at 4♠.

Three-level preempt

In third seat East might just overlook his other four-card major and his overstrength points holding and start with an ostensibly preemptive 3♥.

South will either bid 3NT, or double, leading to a final contract of 3NT or 4♠ respectively.