Another way of showing which Kings you hold has currently become popular with many tournament players.

After a 4NT (Roman Key Card) asking bid and response, when the person who has done the asking then bids 5NT (guaranteeing that all key cards are held) responder should bid as follows :

If are trumps :
Bid 6♣ to show no kings
Bid 6♦ to show K♦ or K♥ & K♠
Bid 6♥ to show K♥ or K♦ & K♠
Bid 6♠ to show K♠ or K♦ & K♥

If ♦ are trumps :
Bid 6♣ to show K♣ or K♥ & K♠
Bid 6♦ to show no kings
Bid 6♥ to show K♥ or K♣ & K♠
Bid 6♠ to show K♠ or K♣ & K♥

If ♥ are trumps :
Bid 6♣ to show K♣ or K♦ & K♠
Bid 6♦ to show K♦ or K♣ & K♠
Bid 6♥ to show no Kings
Bid 6♠ to show K♠ or K♣ & K♦

If ♠ are trumps :
Bid 6♣ to show K♣ or K♦ and K♥
Bid 6♦ to show K♦ or K♣ & K♥
Bid 6♥ to show K♥ or K♣ & K♦
Bid 6♠ to show no Kings

This may look complex but can easily be summed up as :

If you hold a King, bid the suit in which you HOLD THE KING.  It either shows the King or it shows the OTHER TWO KINGS (excluding the suit you bid and the trump suit).

With no Kings, bid 6 of the trump suit.  Remember IGNORE THE KING OF THE TRUMP SUIT, you have already shown it when using Key Card Blackwood