The Three Musketeers of effective Slam bidding
Multi Cues - Roman Keycards - Splinter raises

Ex.1
You open 1♠, your Partner raises to 3♠. You have not that many points, but 6 losers only and therefore ask for Aces (3NT).
Your Partner shows 1 Ace, with your 2 that makes 3 in total, so you bid 6♠.
Partner's hand goes down as Dummy and the Opponents win two ♣ tricks before you even get a look in. A disaster !

So what should you have done ? Next time the bidding goes exactly the same, your Partner's hand fits perfectly and making 6♠ is a walk in the park.
Ex.1
Opener
♠ A J 10 9 2
♥ A Q J 7
♦ K 4
♣ 7 3
W E
1♠ 3♠
3NT* 4♦#
6♠ Pass

Responder
♠ Q 7 5
♥ K 8
♦ A Q J 5
♣ Q J 8 6

a. Has Partner a control in ♣ ?
b. Has Partner Trump K and Q ?
c. Has Partner any Aces ?
Opener should ask Partner three questions (as shown on the right), and simple Blackwood will not provide the answers.   Question a. is the most important.
Partner can have all the points in the world, but with no control in ♣ a Slam contract is not on.

Multi Cues provide the solution for this :
After a trump suit agreement has been reached, the bid of a new suit (above 3 of the major trump suit or above 3NT for the minors) shows 1st or 2nd round control in that suit.
Suits are bid up the line, skipping a suit denies control in that suit

Such control is an Ace or a King or a void or a singleton.

Ex.1
Opener
♠ A J 10 9 2
♥ A Q J 7
♦ K 4
♣ 7 3
W E
1♠ 3♠
4♦* 4♠#
Pass

Responder
♠ Q 7 5
♥ K 8
♦ A Q J 5
♣ Q J 8 6

* = Control in , but no control in ♣
# = No ♣ control either, so no point going on.

Roman Keycards
After the Multi cue bidding has assured that the two hands contain no coinciding weak suit, asking for other controls can go ahead.
Prefer to use Roman Keycards Blackwood.
In addition to Aces it also can reveal Partner's Trump King and Trump Queen.
Keycards are the 4 Aces plus the Trump King.
After a 5♣ or 5♦ Reply, the next bid asks for the trump Queen.
4NT - 5♣ = 1 or 4 keycards
4NT - 5♣ - 5♦ - 5♥ = no Trump Q
4NT - 5♣ - 5♦ - 5♠ = Trump Q

4NT - 5♦ = 0 or 3 keycards

4NT - 5♥ = 2 keycards, but no Trump Q

4NT - 5♠ = 2 keycards plus Trump Q

Ex.2
Opener
♠ A Q 10 9 2
♥ A Q J 7
♦ K J
♣ 7 3
W E
1♠ 3♠
4♦* 4NT#
5♠## 6♠

Responder
♠ K J 7 5
♥ K 3
♦ Q 10 2
♣ A Q 9 6

* = Control in , but no control in ♣
# = RKC ask, no other lack in controls
## = 2 Keycards + trump Q
Ex.3
Opener
♠ A Q 10 9 2
♥ A Q J
♦ K 4
♣ 9 7 3
W E
1♠ 4♣*
4NT 5♥#
6♠ Pass

Responder
♠ K 9 7 5
♥ K 8 5 2
♦ A Q J 5
♣ 6

* = Splinter, 4+ trumps, singleton or void in ♣
# = 2 Keycards, no Trump Queen

The Splinter Raise (Ex.3 above)
This shows 4+ trumps, 11+ HCP and a singleton or void in the suit bid (here ♣).
This is an important control bid. Opener now knows his weakness in Clubs is covered and can immediately proceed to ask for Partner's Roman Keycards.
High cards in Partner's Splinter suit (other than the Ace) are wasted. They should be located in the other suits. In such case a Slam contract is usually unlikely.