A deal from an online duplicate.
After North rebid 2NT in the above auction showing a balanced 18-19 South decided that North needed very specific cards for a slam and settled for game.
Of course North has the perfect hand with good controls and no wasted values and a doubleton diamond so that twelve tricks looks very likely on reasonably friendly distributions.
However this is matchpoints and there is a chance for thirteen tricks; if the hearts break 4-3 you can set up a long heart trick with careful play.
On a diamond lead declarer won in dummy and played ace of hearts and a heart ruff, then a diamond to the king and another heart ruffed with the nine. Next a diamond ruff in dummy, then a spade to the ace and a second spade to the king. When spades broke 2-2 the rest were easy, another heart ruff set up the ten of hearts and the ace of clubs in dummy provided an entry to cash the heart throwing a club from the South hand.
About half the declarers in four spades made thirteen tricks, most of the rest only made twelve. Nobody bid the slam.