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 Unusual Notrump Convention can provide a way to bid two suits at the same time.
After an opponent opens 1 of a suit, a jump to 2NT by you would be a very "Unusual" notrump bid.
This bid shows that you have the two "lowest-ranking" other suits:
  • over a Major suit it shows 5-5 in the minors
  • over a minor suit it shows 5-5, ♥s and the other minor
There is no strength requirement associated with Unusual Notrump, 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.

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