A double, primarily for take out, to convey that the bidder is unwilling to pass but has no good descriptive bid to make. South’s double is competitive in the sequences below.
In the first sequence, it would be pointless for South to attempt a penalty double of a bid that is natural and forcing, especially at such a low level. The double suggests a doubleton heart (not enough hearts to raise to 2♥), a four- or moderate five-card diamond suit (not enough diamonds to be bidding 2♦) and a weakness in one of the black suits (hence the inability to bid 1NT).
This hand would be typical:
In the second sequence, after the opponents have bid and raised spades, it is most unlikely that a worthwhile penalty is available from defending at this level. The double suggests a doubleton heart (not enough hearts to raise to 3♥ or 4♥), five clubs (not enough clubs to bid 3♣) and no spade stopper.
This hand would be typical:
Nowadays, players use many low-level doubles as competitive rather than penalty.
