You hold: ♠KJ7 ♥J3 ♦AK105 ♣KJ83 and your partner opens 1♣. You bid 1♦ and partner jumps to 2NT (showing 18-19 HCP and balanced hand).
Your correct bid is 6NT—you have a combined HCP of 33 or more (and less than 37). NT Slam are JUST adding up the numbers.
Despite this basic fact of bridge, the pair that bid the NT slam against us was one of only 9 out of 20 pairs to bid it. That is very sad!!