It IS quite unusual to have seven-card support for partner’s opening bid. When you do it is easy to overvalue your hand, particularly when you have 7-3-3-0 distribution – three-card holdings often contain losers.
This example comes from the Senior Camrose.
Over 1D North might try 5H, Exclusion Keycard Blackwood, but that drives his partnership to slam when there may be two club losers on top. So he started with a splinter raise. South’s hand looked no-trumpy, though his heart stopper is hardly ideal opposite a singleton. When North removed 3NT to 4D he signalled serious slam intention and South started a cuebidding sequence. It was not possible for either player to visualise 13 tricks, and the partnership landed safely in the small slam. There is play for the Grand because South has the jack of spades – the odds are slightly better than a straight finesse, because someone holding the ace of hearts along with spade length might be squeezed – but it is a Grand Slam you might prefer not to be in.
Unless, of course you are John Matheson and Willie Coyle, playing on this occasion for the sponsor’s Great Britain team. They made short work of the auction: Coyle opened a “Mexican 2D” showing 18-19 balanced; Matheson simply ‘raised’ him to 7D, with the extra chance that West had a blind lead into the strong hand. West did not disappoint: he led the ace of hearts, establishing the king as the thirteenth trick.