It would be embarrassing to bid slam and then lose two aces off the top. If you think you and your partner have enough points for slam, you should bid Blackwood to make sure you are not missing two aces.
         Here's how it works: to ask your partner how may aces he has, simply bid 4NT. Your partner will respond as follows:

иии — 4NT — 5:     0 or 4 aces.
  5: 1 ace.
  5: 2 aces.
  5: 3 aces.

NOTE:     The 5 repsonse may seem ambiguous, but in practice you will always be able to tell the difference between a partner who has no aces and a partner who has all of them!


         Asking for Kings.      After your partner reveals his number of aces, you may ask him for kings by bidding 5NT. Such a bid shows interest in grand slam, and should only be made if you and your partner have all the aces. Your partner will respond to your 5NT bid as follows:

иии — 4NT — 5 — 5NT — 6:     0 or 4 kings.
  6: 1 king.
  6: 2 kings.
  6: 3 kings.


         When is 4NT Blackwood?      Essentially any bid of 4NT at any time is Blackwood. There are two major exceptions:

If your partner just bid notrump, then a bid of 4NT is not Blackwood.
If your last bid was 3NT, then a bid of 4NT is not Blackwood.

In the first case, 4NT is a quantitative raise. To ask for aces in such a situation, you should use Gerber instead.

In the second case, you are arguing with your partner over whether you should be in notrump.