Gerber

Gerber is a refinement of the Blackwood convention. It gets around the problem of an innocent response pushing the bidding beyond a comfortable escape level by starting earlier, at 4C. The responses are shifted in turn:

Obviously, king-asking is then initiated with a bid of 5C.

Downsides

You can't use Gerber if Clubs have been agreed or might be the suit you're agreeing to. Use one of the other slam bidding conventions instead.

Some authors eschew Gerber altogether, because it prevents you from using 4C for splinter, trial and sometimes cue bidding.