When you want to compete after the opponents have opened the bidding you have two common methods.
With a single suit you usually Overcall in that suit.
With support for all the unbid suits you usually make a Takeout Double.

The problem hands are those with two suits, usually 5-5 or 6-5. You don't want to Overcall in one of the suits because you might lose a fit in the other one, but you cannot make a Takeout Double with no support for the 'third' suit.

The Michaels Cue-bid Convention can provide a way to bid two suits at the same time.
After an opponent opens 1 of a suit, a cue-bid of that suit by you shows:
  • cue-bid of either minor suit shows 5-5 in the Majors
  • cue-bid of either Major suit shows 5-5, the other Major and one of the minor suits
The Cue-bidder's partner can request the Cue-bidder to identify which minor suit he holds by bidding Notrump at the lowest level.

There is no strength requirement associated with Michaels, just the 5-5 distribution requirement. This means that later bidding must proceed cautiously. It is best if the partner assumes the cue-bidder is quite weak until he finds out otherwise.

Michaels Cue-Bid is used in conjunction with Unusual Notrump.
Combined lesson available.